Simple Radio Receiver Circuits For Beginners
This part of my site features some simple receiver projects ranging from the simple crystal set to a working TRF Regenerative Receiver. There are circuits available for a Solid State version of the audio amplifier and Regenerative Receiver. Also there are valve circuits available for constructors who prefer to use valves. All the circuits use DC voltages between 6 to 12 volt and are absolutely safe for beginners to construct and young children from the ages of 11 to experiment with, provided they are under the supervision of a parent or adult regarding the safe use of batteries and erecting outdoor antennas. We will first start off with a description of the crystal set. Please refer the following circuit diagrams below and video clip links, to see these receivers working before proceeding with the construction details. The crystal receiver is the simplest and oldest receiver that has been around on all corners of the Globe for listening to speech and music broadcasts and people still build them today using modern electronic components. The crystal receiver was also used for listening to England's first ever radio station 2L2 in 1922 which due to funding in those days could only broadcast once a week and at first it was only limited to 25 minuits airtime every Tuesday evening. At first 2L2 only broadcast speech but as funds progressed, concerts and music started being a more regular thing later. The power of this station was only 100W and was limited to a 15 mile range from the transmitter location for crystal sets and 25 miles for valve TRF Receivers such as my Simple 2 Valve Regenerative Receiver . Back in those days and even now the crystal set can not be strictly described as a portable receiver, despite its simplicity such as the circuit described in picture 1. The reason for this is because it relies on a very efficient outdoor antenna system to get the best results because there is no RF or AF amplification and its electrical energy relies solely on the Antenna system. In October 1922 The British Broadcasting Corporation BBC was formed and as a result more powerful transmitters where built and reception on both crystal and valve receivers greatly improved. Valve receivers in those days where very expensive and the valves where limited from only 1 week to a month of there useful life and the batteries had to be charged using a generator as many of Britain's households did not have a public electricity supply until the early 1930s which also meant a regular trip to a garage or radio shop to add to the expense and it was for this reason the crystal set remained popular for all those years. The circuit described in picture 1 is simply a tuned circuit consisting of VC1 the tuning capacitor and the tuning coil L1. The signals that are picked up by the antenna are known as radio waves and travel at 186,000 miles per second. Heinrich Hertz the Germen Psychologist discovered electricity and magnetism can be combined into the unit of frequency from 1 cycle per second which is referred to AC alternating current also used for the transmission of electricity to our homes at 50 Hertz or 50 cycles per second. The diode detector D1 is a modern germanium diode used to rectify these AC waves into a DC direct current form so they can be recovered as audio suitable for driving high resistance headphones or with a matching transformer, low resistance headphones and in some cases if the signal was very good, a speaker could be used but the volume is very limited to a quite environment due to no amplification. The germanium diode D1 is not the original detector, a crystal set first used but the early ones worked on the same principle as I will briefly describe. They consisted of a rock crystal which is basically a small stone of granite or even a small seaside pebble would surface but the performance regarding volume would vary. A thin piece of wire that had to just touch the crystal formed the anode and cathode and was also named as The Cats Whisker. Resistor R1 gives forward bias for the diode to help stability and may not be needed particularly when feeding low resistance headphones or a matching transformer. The crystal set can be improved 1 step up by adding an amplifier to make the audio signals bigger and the earlier form of amplification was an electromechanical device known as the crystavox and took its power from a 6 Volt battery. Picture 5 is a suitable modern IC audio amplifier suitable for this purpose and will also serve for amplification of the shortwave regenerative receivers described later on in this article. Picture 3 is also another simple version of the crystal set but uses a voltage multiplier diode arrangement similar to the pulse counting detector used in all my FM receivers featured on this site and the performance is basically the same as the circuit of picture 1 but has a slight increase in signal strength due to the voltage doubling action and is more recommended for constructors wishing to experiment with loudspeaker reception. The burning question is what wavelengths and mode of voice modulation can a crystal set receive. A crystal set can basically receive all the HF High Frequency Radio Spectrum from 15 Metres 21 MHZ Shortwave to 1500 Metres 150KHZ Longwave. AM Amplitude modulation is commonly used as it is more sensitive. SSB Single Side Band can not be resolved due to fact that there is no form of oscillation in a crystal set such as regeneration or a BFO Beat Frequency Oscillator and even if they can be picked up, they would be too weak in anycase. The last question is can the crystal set be used for the reception of VHF/FM transmissions. Courtney to popular belief an FM receiver does not have to be complicated like my other designs featured elsewhere on this site but as I said earlier on about the crystal set requiring the best antenna configuration, you need to be almost very next door to the transmitter site and even if that, you may not get the same results regarding volume as you would with medium or shortwave transmissions as FM needs a far more stronger signal to resolve its carrier and slope detection must be used to recover the signal. That is why an FM superhet receiver needs more IF intermediate stages then a MW broadcast receiver. I will briefly describe my own experiences of trying FM reception with a crystal Set to give you an idea of what you may be in for. Using the simple circuit described in picture 1 I replaced the L1 tuning coil for a VHF airspaced coil of about 10 mm diameter which consists of about 4 turns of bare copper wire that is used for the mains earth in UK ring mains circuits. I then padded the 350PF tuning capacitor using 10PF series trimmers to swing the capacitance down to around a low value suitable for the VHF Range. I then linked up my outdoor antenna system along with a straight crystal earpiece. I did at this stage have no success in picking up anything but when I linked up the audio amplifier described in picture 5 I could just faintly hear my local station, Stray FM but was not very audible. This may also be because I was using the OA91 standard germanium diode which is not very good for detecting such high frequencies within the VHF range but I have heard that the 1N34 germanium diode works better and may be tried in the near future. An excellent website composed by Andrew R Mitz at the following address FM only: Low Tech FM Radios has more information regarding the FM crystal set and also some Good Superregenerative FM designs similar to the sets I am going feature in the later part of this article.
Please left click on selected picture to enlarge image.
Please Refer To The Following Video Clip Links To See These Receivers Working
YouTube - My Experimental Loudspeaker MW Crystal Radio.MP4
YouTube - My Shortwave Version Of The Loudspeaker Crystal Set Working On The 41 Metre Band.MP4
YouTube - My Amplified Multiband Crystal Set Driving An LM380 Audio IC Amp.MP4
Safety Guidelines Regarding The Erection Of Outdoor Antennas And Earth Connections
The crystal set is the simplest and safest radio receiver to build as it needs no electric current other then an efficient antenna preferably outdoors and in fact anyone with no previous electronic knowledge can build one within an hour. Having said that, there are 3 simple safety rules that do exist and that involves the safe erection of the outdoor antennas. please refer to the following guidelines below and then your experience of listening to free power radio should give you many happy hours of enjoyment.
1. Picture 8 is the best outdoor antenna arrangement and its support can be a high tree or a 20 foot mast. Insulated tinned copper wire is the best to go for and must be strong enough to withstand the worst of weather conditions such as the heavy wintery gales we mostly get in these typical European Countries. Do not use the bear stranded copper wire that you get with these so called shortwave antenna kits as due to progressive corrosion they lose there efficiency within a week and this was in fact a weekend household task in the early days of crystal radio listening which consisted of lowering the antenna wire supported by a pulley and cleaning off the corrosion with steel wool. Also the use of porcelain insulators for the support wire are available from a DIY shop will make your outdoor antenna system most effective as the signal will not be shorted to ground during damp or wet conditions. If you are still lucky enough to own the old GPO telephones with the analogue dial then the dial stop will make the best antenna you could possibly go for as it is connected to at least 1 conductor of the main BT line, providing it does not interfere with you or any of the household who may need to use the phone.
2. A very important safety rule that must always be obeyed at all times and that is overhead electric pylons. If you live in the countryside such as in a rural farmhouse then the chances are that 11000 Volt power lines normally accompanied with a 240 Volt step-down transformer exist. If this is the case and these pylons happen to run through your back garden then erecting an antenna could result in very dangerous circumstances. The least that can happen if your antenna wire comes within contact with an electric pylon is you could be badly burned or electrocuted and worst of all you and your home could end up being a pile of ashes and it will happen so quickly it will be too late to even know anything about it.
3. Another important safety rule that must be obeyed when necessary is Thunderstorms which exist mainly after a long hot summer spell. You must refrain from using your outdoor antenna system at all times when these sort of weather conditions exist as this can result in similar circumstances such as mentioned in rule 2 if your antenna system happens to be struck by lightning. You must immediately disconnect your antenna and put it well out of reach when these conditions exist and carefully explain to your child in a firm but nice way to why he must not use the crystal set when there are thunderstorms present or forecast.
Issues Regarding Earth Or Ground Connection
The crystal set also relies on a decent earth connection but only on the medium and longwave bands. As you get into the real shortwave bands around 6 MHZ 49 Metres the earth connection becomes less effective but a short wire of about 10 metres connected to the earth and strung around a room can improve reception as this acts as a double dipole antenna. Picture 9 is the best outdoor earth arrangement which consists of a copper pipe about 1 metres long driven into the ground. The connection must also be cleaned periodically normally after a wet or heavy cold spell as the connection will lose its conductivity due to corrosion. There is 1 more important safety rule regarding the use of alternative earths within the home which I will briefly describe.
4. The use of a water or central heating pipe is the next alternative if installing an outdoor earth is too much of a pain but never a gas pipe or the earth of the mains electric wiring system within the home for the following reasons.
5. Courtney to popular belief a gas pipe is not dangerous provided the pipe is not accidently damaged when installing the earth connection but your local gas company may not approve of it and may ask you to remove it or worst of all they could take legal action against you for tampering with there apparatus.
6. Never be tempted to use the earth of the electric ring mains which involves strapping the earth connection to the metal case of an electrical appliance or using the earth prong of a mains plug. The obvious reasons for this is because the mains wiring within the appliance could be faulty and a severe electric shock hazard may exist. Also the later idea of using a mains plug is more dangerous because it may fall into the wrong hands of somebody modifying the connection to the L live terminal which could result in a fatal electric shock.
7. On a closing note, If the above simple rules are always followed, crystal set building will provide you with many happy hours of fun and the joy of listening to a radio broadcast many thousands of miles away from you without the use of electric current of any kind can be a real fantasy dream.
Construction Details Of The Crystal Receiver
If you have read all the above details concerning the antenna erection details you can now go ahead with the crystal receiver of your choice. Please note that unless you live in a good reception area such as open countryside the crystal set will give very poor results or not work at all so consider erecting a decent antenna system before going ahead with the construction of your set. Even if you outgrow from using the crystal set, the antenna will serve for long distance shortwave reception with any regenerative or superhet receiver that you may intend to build later on. Just a brief note regarding the pictures of the 2 portable loop antenna versions of the crystal receivers featured in pictures 10 and 11. The Medium Wave version will work with quite acceptable results if you happen to live very close to a local MW station but not as good as the versions using the long wire antenna method. The Shortwave version has mixed results. It will either not work at all or if band conditions are very good stations can come in very loud and it is possible in a quite environment to even use a speaker provided a matching low impedance transformer is used. The straight single diode detector crystal set described in picture 1 is the easiest one and is more recommended for the absolute beginner to build before moving on to an advanced design. Please note that the crystal set project can be very experimental and even if you wire it up wrong there is no danger of damaged components as there is no electric current of any kind involved. Also as there is no regeneration or amplification involved other then the energy from the antenna, no instability such as oscillation can exist until you decide to add a regenerative detector. Please refer to the details below regarding general construction details.
General Construction Details
Even if you have never had experience with soldering before you can use techniques such as breadboard and wire wrapping and surprisingly quite complex receivers can be made using this technique. As an example I started my own experience using cupped screws on a base board back in 1972 during my early childhood days from the Lady Bird Book, Making a transistor radio and the article can be found at the following link TRF Radios 2, HAC Radio, Ladybird Radio, Denco Green Coils . This site also has some regenerative circuits similar to the the designs that I am going to feature later on in this article. Soldering is obviously the best method and all your receiver projects weather transistor or valve will work more stable due to more secure and rigid connections. Young children must be supervised at all times when using a soldering iron. The low voltage heat controlled soldering iron with an isolation transformer is best regarding safety when young children are experimenting and it must have a minimum temperature of 25W. A suitable bench or old table can be used. Also lead Free solder is more hygienic but unfortunately does not give as good quality shiny soldering joints as the basic resin core solder. A protective mask designed for this particular purpose is advisable. For anyone who has never had experience of soldering before then you must visit the following 2 links EPE "Basic Soldering Guide" and EPE Basic Soldering Guide Photo Gallery as these 2 sites have all the necessary tuition guides to the wrong and right ways if you wish to learn how to solder.
Construction Of The Tuning Coils
Please refer to picture 7 for full construction details of all the shortwave and medium wave coils that have been found to perform well with any version of the crystal receivers featured. Also they can be easily modified for use with the regenerative circuits although an extra reaction and preferably an aerial winding for better selectivity will be required. The medium wave coil 550KHZ to 1600KHZ is wound on a 80mm X 20mm diameter wooden dowel former and consists of 100 or 120 turns of 36 SWG of enamelled copper wire wound side by side. Double sided Vero pins or small tack nail pins can be used to support each terminal. Each end of the enamel must be scraped clean before wrapping the wires around the pins and then they can be tinned with solder to ensure good connectivity. The SW1 1.5MHZ to 4.5 MHZ coil is wound in the same way but is wound on a 70 X 20mm wooden dowel and consists of 55 turns of 26 SWG enamelled copper wire. This coil covers the top end of the medium wave band and the 2 tropical short wave bands 120 metres 2.3MHZ to 2.498MHZ at the low frequency end of the range. 75 metres 3.95MHZ to 4.0 MHZ is also possible at the high frequency end of the band. This particular coil is not the best choice for crystal sets as these stations are low powered but when DX condition are favourable such as late winter afternoons to early evenings, it is possible to get some quite interesting results from these frequencies. SW2 is the best choice of all as it covers 7.5MHZ to 15.0MHZ. It also includes the 41 metre and 31 metre Broadcast bands which are very active during the late evenings and is the easiest coil to wind. The SW2 coil is also wound in the same way as the other coils and is wound on a 35mm X 20mm wooden dowel and consists of 8 turns of 20 SWG enamelled copper wire wound side by side. It is also possible to cover the higher frequency end of the shortwave spectrum that is 16 metres 17.55 to 11 metres 26.1 MHZ but the sensitivity will be to low and that is archived by taking around 3 turns off the SW2 coil.
Construction Of The Basic Crystal Receiver
1. Please make sure you have all the necessary to hand before constructing the receiver of your choice by clicking on the following link Components List Of The Crystal Radio before proceeding.
2. Please refer to picture 1 the circuit diagram and picture 2 the wiring diagram if this is the particular receiver you intend to build. This version is more recommended for a first time receiver. If you wish to build the voltage doubler version of the crystal set then you need to refer to picture 3 for the circuit diagram and picture 4 for the wiring diagram.
3. If building the the receiver on plain Vero board the leads marked in orange pencil route over the circuit board.
4. Do take special care when soldering the germanium diodes in all versions of the receiver insuring that you don't apply too much heat. The polarity of D1 is not important when used as a straight crystal set but if you intend to link it to an audio amplifier later on, you must make sure the red + positive end corresponds correctly or the sensitivity may be reduced if using an alternative amplifier for the amplified version of the crystal receiver featured in picture 5. If you are using the Voltage doubler version of the crystal set featured in pictures 3 and 4 then the polarity of the diodes is very important or the voltage doubling action will not function and the receiver may have very low sensitivity or not work at all.
5. When using a crystal earpiece or high resistance headphones the audio matching transformer is not required and same applies when linking the crystal set to an audio amplifier unless the input is a low impedance input designed for magnetic phonograph pickups.
6. If using the low impedance type of 8R earpiece that is designed for use with transistor radios or the Sony walkman type headphones then the T1 Audio Transformer has to be used or the volume will be very low no matter how high the signal level entering the antenna is.
Special Issue Regarding Loudspeaker Reception
When environmental conditions are quite such as in the early hours of the morning and providing you have a very good antenna system, it is possible to archive loudspeaker volume with a crystal set but please note the following. As there is no amplification, don't expect to have a free all dancing radio that can be used at a party or when a family of say four people are present in the kitchen having a meal. The matching transformer will be essential as all speakers are 8 or 16 ohm specification and the larger the speaker, the louder the volume will be. The volume is also only usable on the very powerful stations.
Testing And Operating Your Crystal Receiver
Operating the crystal receiver is very simple and as there is no on/off switch to worry about it may be already working almost the moment a decent antenna is connected along with some suitable headphones.
1. Connect a suitable antenna to the SK1 Antenna terminal if using a short antenna.
2. Connect a suitable earth or ground to the SK3 Earth terminal.
3. Connect a suitable pair of high impedance headphones or the specialised matching transformer if you intend using low impedance phones or a loudspeaker.
4. Now for the moment of truth. By rotating VC1 it may be possible to at least tune in 2 or more high power stations within your region.
5. If the high power stations are untunable, then your antenna is too long and you need to swap over to the SK2 antenna socket which is connected in series with the antenna tuner VC2.
6. Closing the vanes of VC2 highers the capacity of VC2 which needs to be at this setting when signals are weak.
7. Opening the vanes of VC2 lowers the capacitance and will help separate the strong signals by attenuating the signal and some of the weaker ones will also be heard. Also this method will match your antenna more efficiently although a coupling winding over the antenna coil is a better alternative and will be used with the regenerative circuits.
7. Hope you have many happy hours of using your crystal receiver and it should learn you a great deal on how the radio receiver works before progressing to advanced circuits such as the audio amplifier that will be the next step up from the crystal set.
Constructing The Audio Amplifier To Form An Amplified Crystal Set
As the crystal set is obviously your first working receiver and presuming you have managed to get the project working ok I can not see why you should not progress further afield towards building this simple IC audio amplifier which will greatly improve the volume of the crystal set and will serve for the TRF and regenerative receiver circuits later on. Please refer to picture 5 which is the circuit diagram and picture 6 the wiring diagram of the amplified crystal set. The LM380N Audio IC used in this circuit has been around since the early 70s and was used in cheep record players and music centres of that era. It has a maximum output of about 2 Watts audio power when used with an 18 Volt supply but as we are only using a 9V battery supply the power will be limited to around 500mw which is more then enough volume for this particular application. If treated with care, it is a very reliable audio IC and also features short circuit protection and thermal shutdown to protect this IC. Before proceeding with this circuit and that also includes the regenerative circuits that will follow, please take the time to read these simple precautions related to the safe use of alkaline batteries and the polarity of certain components.
General Safety Precautions Regarding The Safe Use Of Alkaline Batteries And Low Voltage Circuits
As these circuits use a safe low voltage of no more then 12 Volts there is no serious electric shook hazard involved but please read these simple rules regarding the safe use of batteries and the polarity of certain components, then hopefully you will gain further enjoyment in building these other receiver projects. As dry cell batteries are no longer available we all have to resort to using high power alkaline batteries. The good advantage is they have a longer life span but the downside of them is that they are capable of delivering more excessive currents and are not really suitable for young children to experiment with unless supervised by an adult. Please read the following guidelines below before proceeding with the amplifier circuit.
1. Avoid shorting the terminals of these batteries as a continues short circuit will cause them to heat up and possibly explode which could also be a fire hazard.
2. As we all know, solid state circuits are nowhere as rugged as valve circuits so you must double check the polarity of certain components and general wiring before connecting up the battery supply or permanent damage may result.
3. You must at all times pay attention to the polarity of electrolytic capacitors as these can heat up and explode causing there innards to deposit all over the place. Most electrolytic capacitors have a continues stripe marked down the can for the negative terminal and you may notice it being a shorter lead.
4. Also as a final precaution, double check the battery polarity and it is advisable to have a switch in the positive supply line so the circuit can be switched off when not in use and also helps an accidental brush with the wrong polarity when changing batteries as this can also cause permanent damage.
General Construction Details Of The Audio Amplifier Circuit
1. Please make sure you have all the components to hand before attempting to construct this version of the receiver by visiting the following link Components List Of The Crystal Radio and refer to pictures 5 and 6 for the circuit and wiring diagram.
2. If you are now confident you have followed all the above procedures you may now go ahead with the basic construction of this simple amplified crystal radio circuit. A 14 Pin DIL socket is advisable for IC 1 as excessive heat can damage this IC when soldering. NB There is a notch on this IC which corresponds to pins 1 and 14 which I forgot to include in the circuit diagram and I will update it at a later stage. If this sounds confusing, then the amplifier for my Solid State AM/FM Pulse Counting Receiver Designed For HF Short Wave Band Reception As Well As Superb Clear VHF/FM Reception is exactly the same version and can be copied from the following link wiringdiagramoflm380naudioamplifier.jpg until I rectify this mistake.
Testing The Amplified Crystal Set
1. If you are confident that you wired everything correctly then you may give it a first time test but it may be advisable to double check again, particularly if this is your first time electronic project. Make sure in particular that there are no dry solder joints or whiskers that are a common cause of short circuits and a magnifying glass is a very handy tool for checking these most common mistakes.
2. Connect a suitable 8 Ohm speaker to the amplifier output and a suitable antenna to the SK1 Antenna terminal. NB If it is a very long outdoor antenna you are using you may have to use SK2 or receiver overload may result and the selectivity will be very poor.
3. Connect a suitable 9 Volt supply, making sure you observe the correct polarity and switch on for the great moment of truth.
4. If all is well, you should at least be able to hear some sort of activity such as a soft hiss in the speaker and touching the positive side of the D1 diode detector should produce a buzz when the VR1 volume control is advanced.
5. By rotating the VC1 tuning capacitor, it should be possible to tune in a few stations in the same way as the basic crystal set but with much greater volume. Also the operation is very much the same as the other crystal set circuits.
6. This now completes this section regarding the crystal set project and I am pretty sure you will be wanting to move on to a more advanced regenerative receiver project. If you have already outgrown your crystal set then the VC1 and VC2 Tuning capacitors will serve for this along with the LM380N audio amplifier circuit.
Simple Regenerative TRF Receiver
This section features a regenerative receiver capable of receiving all the Medium wave to the entire HF Shortwave radio spectrum between 550 KHZ to 30 MHZ. There are 3 versions of this receiver. The first version is a simple 1 transistor receiver that will just drive the LM380N audio amplifier when signals are strong and is suitable for an absolute beginner to construct who has had previous success with the crystal radio circuits. The second version features simple band switching and a tuned or untuned RF stage can be added. The third version features an input audio pre amplifier which will improve volume and make weak stations more hearable. I will give you a brief description of how the Regenerative Receiver works and its history. Any Valve or Transistor can form as a detector for rectifying RF signals into audio in the same way as the crystal detector by using what we call leaky grid detection or anode bend detection and can also function as an RF Amplifier at the same time. Also more tuned stages can be added ahead to form what we call a TRF Tuned Radio Frequency Receiver. Although using this method can make the receiver more selective and have more gain, It is more costly to build as each RF Stage requires an extra tuning capacitor and unless they are all ganged together it would be a very difficult receiver to use as each tuned circuit needs to track at the same received frequency. This is why the later Superhetrodyne receiver eventually gained popularity and still to this day, It is widely used for all forms of radio communications. The great inventor Edwin Armstrong invented the Regenerative Detector in 1914 before moving onto the invention of the now widely used Superhetrodyne receiver which many radio technicians like myself call it the Superhet for short. To make an RF amplifier regenerative you need to amplify the output signal back into its input. In the most simple case it can be done by using a capacitive method which is also used in very simple regenerative receivers by connecting a capacitor between the anode and grid of any Triode or Pentode valve in which we now have what we call Positive Feedback. In early valve days it was also called Reaction but is now described in modern terms as Regeneration and as a result you can increase the gain of an RF amplifier by a factor of 15,000 or more. Applying too much Regeneration causes the RF amplifier to go into oscillation and is described as a squalling noise which can make the signal you are listening to unintelligible or cause illegal interference to nearby receivers which are tuned to the same frequency, So we need to find some way of bringing this situation under control in which I will give you an example. By referring to picture 3 which is the circuit diagram of my 1 Transistor Regenerative Receiver you need to note the following points. L1 is the main tuned circuit which is simply tuned to the frequency of interest and is capacitive coupled to the Gate input of TR1 with R1 forming bias. L2 is what we now call the feedback or regeneration coil. The signal is now fed back from the Drain of TR1 through the coupling capacitor C2 which also blocks the passage of DC and only allows AC signals to pass. The signal is then coupled to L2 which also must be inductively coupled in the correct phase which means having the windings of L1 and L2 in the same direction of the Magnetic Field or Regeneration will not work. The RV1 Potentiometer control is the KEY to bringing this Regenerative Detector under control which I will briefly describe. By rotating RV1 in the anticlockwise direction you are directing the track of RV1 in the earthy direction which also reduces the RF Radio Frequency Current to L2 resulting in less positive feedback and we now have a usable Regenerative Receiver. The receiver is at its most sensitive when RV1 is set just before the freshhold of oscillation and you will have to maintain this control when retuning the receiver to different frequencies which is the main disadvantage of the regenerative receiver. Another popular method of regeneration control is to have C2 as a variable capacitor and RV1 omitted but it is in my opinion the less preferred method for obvious reasons. Firstly, You have the increased cost of buying another tuning capacitor and it also results in detuning effects and backlash. Also many constructors that have built valve versions of this receiver have experienced burned out feedback coils and HT load resistors when using the variable capacitor method of regeneration due to the simple fault of the capacitor vanes shorting to each other. Despite the drawbacks of this simple receiver, It is 1 learning stage up from the Crystal Set and you should have many happy hours of fun, Before going on to more adventures in the world of Wireless.
Please left click on selected picture to enlarge image.
Please Refer To The Following Video Clip Links Below To See These Regenerative Receivers Working
YouTube - My Solid State FET Version Of The Regenerative Receiver Working On MW.MP4
YouTube - My Solid State Version Of The FET Regenerative Receiver Working On Shortwave
Construction Details Of The 1 Transistor Version Of The Regenerative Receiver
Before commencing with the construction of these featured Regenerative Receiver projects it is very advisable to refer to the following link Components List For All Versions Of The Solid State Regenerative Receiver which will have a list of all recommended component suppliers.
Important Guidelines Regarding Safety And General Construction Layout
As I presume you have read the safety article Relating to the Crystal Set, The rules are exactly the same regarding the safe use of Antennas and the polarity of certain components such as the electrolytic capacitors and Batteries if you have built the amplified crystal set. Do pay special attention with the polarity of the FET Field Effect Transistors. The 2N3819 FET is not as static sensitive as other expensive MOSFET devices used in other applications such as high power PA Amplifiers, But you must try to handle it as little as possible and try not to apply no more then 5 seconds of heat when soldering into the circuit which should be just enough to make a nice clean shiny joint. Veroboard is suitable for the audio amplifier circuit, But I do strongly recommend the old style of wiring which is the use of laminated PCB board, Because it forms an excellent ground plain resulting in less hand capacity effects when tuning in the higher shortwave frequencies. Also do not overlook the advantage of using the valve style method such as aluminium cases and tag board.
General Construction Of The Receiver
Please refer to picture 3 which is the circuit diagram and picture 4 the wiring diagram. The copper cladboard specially recommended for all these versions of the regenerative receiver is available from Maplin Electronics code WF41U and should accommodate all hardware and components. The RF coils weather band switching is used or not are best mounted on the left hand side with the tuning capacitor in the centre. Also if band switching is used you must mount each tuning coil about 2 inches or 40 mm apart to avoid coupling effects and make sure that you keep wiring in the tuned circuits as short as possible, Particularly the SW3 high frequency coil. The other wiring is not as critical but do try to keep it as neat and short as possible. All Wiring of this receiver is supported by tagboards and points marked E are chassis earth return points in which all leads can solder direct to the copper board. There is one exception regarding the LM380N audio IC Amplifier chip. It must be mounted on veroboard and a 14 pin DIL holder is recommended to avoid the risk of damaging this IC when soldering. Also you must make sure this IC is wired or plugged into the holder with the correct polarity or it may be immediately destroyed. A notch and small moulded dot at the top confirms the correct polarity. The TR1 FET Transistor must also be wired into the circuit in the correct orientation or this component will also be damaged. It is a half moon shape and its lead-out base is viewed at the bottom as you can see in the wiring diagram. It is best to mount all components such as the tuning capacitor and all other components such as controls before commencing with the general wiring of the receiver. A soldering Iron with a minimum rating of 25W is recommended and leaded resin core solder is recommended for clean reliable soldering. This receiver can also be tested before connecting the tuning coils as they are all capacitive coupled rather the forming part of the biasing network which is the case when using source or cathode feedback which would immediately damage the FET Transistor due to an open circuit. If you confidently understand these procedures you can now go ahead with the construction of this receiver. Before testing this receiver double check the polarity of all components such as the FET Transistors, Electrolytic Capacitors and IC1.
Testing The Receiver
1. If you are confident you have carefully gone through all the above procedures you may now test this receiver before going ahead with winding the RF Tuning Coils.
2. Connect a suitable 8 Ohm Speaker to the SK4 and SK5 Speaker output of about no lower then 500mw rating.
3. Connect a suitable 9 Volt power supply, Preferably 6 AA 1.5V cells making sure you observe the correct polarity.
4. If all is well you should now hear a soft hiss in the speaker.
5. Advance the RV2 volume control fully clockwise.
6. By touching the Gate of TR1 or the stator moving vanes terminal of VC1 you should now hear a loud pitched buzz. If this is so, You have now got the receiver working correctly and it is now down to winding the tuning coil of your choice to start enjoying short wave reception on your first regenerative receiver project.
Winding The RF Tuning Coils
The RF tuning coils are home made in a similar way to the crystal set and are a bit more complex because 2 extra windings are required in addition to the main tuned circuit. 15mm diameter plastic tube is used and is available from most DIY Stores. If you happen to be lucky and have an old set of the Denco type plug in coils then it will save you a job with coil winding. Please bear in mind that these coils are no longer available but as this type of receiver can be experimental it is not that difficult as it seems provided you follow the later instructions very carefully.
1. Please refer to picture 11 which shows you the physical details of how these RF coils are wound.
2. You must also make sure that the tuning coil L1 and L2 Regeneration Coil are connected in the correct phase as shown in the diagram or regeneration will not be possible. All windings must also be in the same direction and although the antenna coil L3 is not as critical, It still must be connected as shown or there will be a reduction in sensitivity. Also to save winding L3, An antenna may be connected direct to the hot end of the tuning coil via the TC1 Trimmer capacitor although bear in mind that this is not the best method when using long antennas unless you have lose enough capacitance as it can be responsible for unreliable regeneration when tuning certain parts of the band concerned and also signal overload that reduces selectivity.
2. Please refer to the information table below for coil winding details for the band of your choice.
Medium Wave 550KHZ - 1600KHZ
L1 Tuning Coil 130 Turns
L2 Regeneration 30 Turns
L3 Antenna Coil 20 Turns.
36 SWG enamelled copper wire is used and turns are wound side by side.
SW1 1.6 MHZ - 4.0 MHZ
L1 Tuning Coil 60 Turns
L2 Regeneration 15 Turns
L3 Antenna Coil 15 Turns.
26 SWG enamelled copper wire is used and turns are wound side by side.
SW2 6.0 MHZ - 15 MHZ NB Best band choice as a starter for getting the receiver up and running as this frequency range has 24 hour European coverage
L1 Tuning Coil 13 Turns
L2 Regeneration 8 Turns
L3 Antenna Coil 4 Turns
20 SWG enamelled copper wire is used and turns are wound side by side.
SW3 9.0 - 30.0 MHZ
L1 Tuning Coil 6 Turns
L2 Regeneration Coil 4 Turns
L3 Antenna Coil 2.5 Turns.
20 SWG enamelled copper wire is used and turns are wound side by side.
Special Notes Regarding Bandswitching
Simple band switching is possible with this type of receiver and is achieved by using the simple switching circuit described in picture 5. For first time constructors to this receiver it is advisable to get it working as a single band receiver first, To ensure all frequency band coils work correctly and that you have gained enough experience of how the receiver works before making modifications. You must also make sure you keep all RF wiring to the band switch as short as possible without the insulation of each lead touching another as this can lead to stray capacitances. One final note regarding the subject of bandswitching. You must also make sure that the SW3 coil has the shortest route of wiring to the bandswitch and all coils must be spaced around 40mm from each other to avoid undesired stray coupling that can also cause a considerable reduction in performance.
Initial Testing Of This Receiver
1. Connect a short antenna of about 4 metres of insulated wire as a start to the SK2 antenna socket.
2. Advance the TC1 Antenna trimmer vanes around mid setting.
3. Advance the RV2 Volume control clockwise to around mid to full setting.
4. As a final note double check all wiring and reconnect the battery, Making sure you observe the correct polarity.
5. By rotating RV1 clockwise, The set should break into oscillation which means everything should be working OK.
6. Slowly advance RV1 anticlockwise until the oscillation just stops.
7. By rocking the VC1 tuning control it should be possible to tune in some station depending on the band you are tuned to regarding the time of day and band conditions.
8. You will find that you will have to keep maintaining the regeneration control as you tune from one end of the band to the other and you must accept that this is normal and is the main disadvantage with the simple regenerative receiver which is why the Superhet Broadcast Receiver became the worldwide consumer standard during the early 1930s.
9. You may find when using SK2 as your antenna source that regeneration can be unreliable and detuning effects occur. If this is so then you need to reduce the capacitance of TC1 by gradually opening the vanes until oscillation of the regenerative detector can be maintained.
10. If you have a long outdoor antenna then the SK1 antenna socket has a big advantage. The receiver is less prone to overloading by strong signals and the regeneration should be more reliable resulting in no dropouts when the antenna is moved. Also the receiver should be more free from frequency detuning effects.
11. If you find that regeneration cuts in to soon, that is with RV1 at its lowest setting or is uncontrollable then the following tips should help. Try removing a turn or so off L3 and this should improve matters. Also decreasing the value of C2 may also help and having the R7 decoupling resistor replaced with a 10K preset may also help matters.
How To Receive SSB Single Sideband And CW Continues Wave Reception
Although a BFO Beat Frequency Oscillator is required when receiving SSB on a Superhet Receiver, The regenerative detector is opposite which is 1 good advantage about this type receiver that also saves the cost of expense compared building a complex communications receiver. Most SSB and CW Morse is listened to on the Amateur Ham HF Bands although shipping, Long range aircraft and coastguard stations still use it and some interesting transmissions can be picked up. Meanwhile if your interest is listening to amateur radio then the SW1 1.6 - 4.00 is the best coil to use as this covers the the 80 metre band 3.5 - 3.8 MHZ which is still a very popular evening chat and long distance DX Band. The 160 Metre 1.8 - 2.0 MHZ known as Top Band can sometimes be picked up but as this band only allows a maximum of 10 Watts of transmitter PA output power it is not as active.
1. A sideband signal is identified by tuning in a garbled and distorted speech when using normal AM Amplitude Modulation.
2. If this is the case you then need to resolve this issue which involves advancing RV1 clockwise until the receiver just breaks into oscillation.
3. By carefully and slowly rocking the VC1 Tuning capacitor within each side of the frequency carrier you should be able to resolve the signal in the form of Donald Duck or someone talking through a drainpipe which is normal when using this mode of transmission.
4. Although this is quite OK to have the detector oscillating when receiving these type of transmissions, Avoid having the receiver in this continues state when in particular using the Medium Wave broadcast band as this may cause interference to nearby receivers which is classed as illegal.
I hope you have managed to build your first working regenerative receiver successfully and this will certainly give you an inexpensive means of listening to the world, Right at your finger tips which is great step up from the simple Crystal Receiver. If you want to improve on the simple 1 transistor regenerative receiver then why not try the 2 transistor version of this receiver which includes an untuned RF stage and the 3 transistor version includes an AF preamplifier which also gives a considerable boost in AF gain.
Construction Details Of The 2 And 3 Transistor Version Of The Regenerative Receiver
The 2 transistor version of this receiver is basically the same configuration as the 1 transistor version except it has 1 slight improvement. It incorporates an untuned RF Amplifier which has 3 advantages. It isolates the antenna from the regenerative detector which can radiate illegal interference when the detector is oscillating and also eliminates unreliable regeneration when tuning certain sections of the band due to tight antenna coupling. It also has a slight increase in gain and sensitivity when using short antennas. It would also be much better to have the RF amplifier tuned, Except this would increase the complexity of the receiver requiring an extra tuning capacitor and coil for the band concerned and would require careful planning of the circuit layout if the risk of stray coupling and instability is to be avoided. The construction and setting up method is the same as the 1 transistor receiver so there will be less comment other then you need to refer to picture 6 for the circuit diagram and picture 7 for the wiring diagram. Also if you wish to incorporate band switching then I have some very good news. It is much easier then the 1 transistor version as there is only the tuning coil winding and the regeneration winding to switch. If You are planning to include band switching then you need to refer to picture 10 which has the full circuit diagram for switching all 4 frequency ranges of the shortwave spectrum. As the RF Stage is untuned it is strongly recommended to include the simple RF antenna attenuator circuit in which you need to refer to picture 12 or the risk of overload and untunable signals may make the receiver more unselective particularly when using a long wire antenna. There is a very useful and simple circuit that has been included which I will briefly describe. It is an accessory that every serious SWL shortwave listener should process. It can be used on all types of shortwave receiver including my Severn Valve HF Superhet Receiver Designed For Advanced Constructors except the AC/DC type where a live chassis exists, As this could pose a serious electric shock hazard. It is called an ATU short for Antenna Tuner Unit and it consists of a series resonant tuned circuit of a tapped switchable coil and parallel tuning capacitor at the antenna input. The purpose of this item is to reduce out of band signals and when tuned properly it effectively matches the antenna which results in a more efficient signal transfer which is responsible for a great increase in gain with less background noise. Please refer to picture 13 if you wish to include the simple ATU circuit. The 3 transistor version of this regenerative receiver is also basically the same as the 2 transistor version except an AF audio input preamplifier has been included which also gives a worthwhile increase in audio gain particularly when receiving very weak stations. If you intend building this version of the regenerative receiver then you need to refer to picture 8 for the circuit diagram and picture 9 for the wiring diagram. As the RF tuning coils are slightly different regarding the text in the circuit diagrams and to avoid confusion with the details for the 1 transistor version please refer to the coil table below.
RF Coil Details For The 2 And 3 Transistor Regenerative Receiver
Medium Wave 550KHZ - 1600KHZ
L1 Regeneration 30 Turns
L2 Tuning Coil 130 Turns
36 SWG enamelled copper wire is used and turns are wound side by side.
SW1 1.6 MHZ - 4.0 MHZ
L1 Regeneration 15 Turns
L2 Tuning Coil 60 Turns
20 SWG enamelled copper wire is used and turns are wound side by side.
SW2 6.0 - 15 MHZ
L1 Regeneration 8 Turns
L2 Tuning Coil 13 Turns
20 SWG enamelled copper wire is used and turns are wound side by side.
SW3 9.0 - 30 MHZ
L1 Regeneration Coil 4 Turns
L2 Tuning Coil 8 Turns
20 SWG enamelled copper wire is used and turns are wound side by side.
This now completes this article regarding the construction of the simple solid state HF Shortwave Regenerative receiver. Many constructors may wonder, Is it possible to use this version of the Regenerative receiver to listen to VHF/FM broadcasts or the 2 metre Amateur Band. Unfortunately because of its lack of sensitivity for VHF use, This type of Regenerative Detector is not suitable but the article below features a simple 3 Transistor Super Regenerative Receiver and is convertible to a simple Superhet Receiver using no coils pulse counting technology.
Simple VHF Receivers
This section features 2 simple VHF Receivers. The first is a simple Superregenerative receiver which I will briefly describe its operation. The Superregenerative detector works in a similar way to the standard regenerative detector except it is more sensitive at working on the higher VHF frequencies and also the most inexpensive way of producing a simple portable FM Receiver. It is also as easy to build as the standard regenerative receiver. Inventor, Edwin Armstrong found by using an interruption oscillating method that is known as the quench oscillator that a signal can be amplified at supersonic rate that is more powerful then the basic regenerative detector. Although Edwin Armstrong was the Pioneer inventor of FM Radio Broadcasting, He did not approve of using the Superregenerative detector for receiving FM broadcasts for obvious reasons which I will Briefly describe. Because the detector is always oscillating at reception frequency which can be identified by a potent background hiss, In the absence of a strong signal they can radiate illegal interference to other listeners unless the receiver is enclosed in a metal box with an RF Amplifier to isolate the Antenna and it was this reason that Armstrong himself did not want his FM service to receive bad publicity. Superregenerative detectors also found there use in simple walke talkie transceivers that are available as toys and model aircraft fans have used them as an inexpensive way of getting there plains to fly but also because of there low selectivity many people had to wait there turn if the frequency was close to an adjacent channel or a model air pileup would result. I will briefly describe how mine version works by simply referring to picture 1 which is the circuit diagram. The first stage TR1 is an FET grounded gate untuned RF stage and its purpose is to isolate the antenna from the detector to eliminate unreliable oscillation when tuning from one end of the FM band to the other. The RF Stage also plays its role in reducing illegal interference from the antenna which is one of the main reasons why this receiver lost its popularity as a simple FM receiver in the early 50s when the Ratio Detection method was introduced in the commercial broadcast superhet receiver. The second stage TR2 is the main ingredient that makes this receiver work. It is a self quenched Superregenerative detector consisting of the Colpitts Oscillator also invented by Armstrong and is very sensitive and capable of receiving the entire VHF Radio spectrum between 30 MHZ to about 200 MHz. Components C5 and C6 are the KEY to making this oscillator work. There values are chosen to make the oscillator work at a supersonic rate of 30KHZ and R4 also determines the quench frequency rate. The purpose of RFC 3 is to prevent the RF signal from being lost to earth which would stop the detector from oscillating and also helps prevent the RF signal from getting into latter audio stages. RV1 is the regeneration control and unlike the standard regenerative detector its adjustment is not as critical and can be left as it is once satisfactory oscillation is achieved. R5 and C7 form the simple quench filter and TR3 is a simple AF stage to make this receiver capable of driving any audio amplifier. Although the Superregenerative detector is still an AM detector, Slope detection is used to recover the FM signal by tuning ether side of the stations carrier frequency. This simple Superregenerative receiver can also be used as a normal regenerative detector in the same way as the simple shortwave receivers which I will briefly describe. By carefully backing off the RV1 regeneration control the reduced voltage reduces the current to the detector bringing it out of supersonic oscillation. When using this later method, The receiver is not as sensitive and adjustment of the regeneration control is more critical. There are 3 good advantages of using the straight regeneration for receiving FM Broadcasts compared to using the Superregenerative detector which I will describe. You are not radiating illegal interference and if you live in a good FM reception area it is possible to get reasonable high quality reception. Also if you happen to have a very good antenna system or the station is very strong it is possible to use this type of receiver for receiving stereo broadcasts providing a suitable stereo decoder is used which will be described in the construction article. This type of VHF Receiver can also be modified to a broadcast superhet receiver using pulse counting technology which I will briefly describe. The Superhet receiver is the next step up from the normal regenerative receiver and became consumer standard in the mid 1930s. A superhet receiver is also more powerful then the regenerative receiver and offers better selectivity then any form of tuned radio frequency receiver weather it is a crystal set or regenerative receiver. If you wish to know more on how the standard broadcast superhet receiver works then visit the following link 3 Valve Regenerative Superhet Receiver . When UK VHF/FM Broadcasting began in 1955 all consumer valve receivers used a 10.7 MHZ IF Intermediate Frequency and ether the Ratio Detector or the more common Foster Seeley Discriminator was used. Both these detectors are not easy to set up and align like the standard AM Diode Detector and are not recommended as first time radio projects for constructors new to radio set building to attempt. This particular receiver we are going to build and set up uses the simplest form of superhet techniques resulting in no IF Intermediate Frequency Transformers to wind and provided you have got all the VHF circuitry working correctly it is easier then a medium wave superhet receiver to align. It is known as pulse counting technology which I will briefly describe and how this FM Detector works. The pulse counting discriminator converts the incoming RF Signal into a train of constant amplitude pulses producing a voltage proportional to the frequency. These pulses are then filtered resulting in the output rising as the pulses grow longer and as its output falls the pulses grow shorter. This method of detection then recovers the original RF Signal that was used to modulate the FM carrier. A voltage doubler diode detector circuit similar to the loudspeaker crystal set is required in all these types of receiver to perform this method of detection and is commonly described as a diode pump charge circuit. As the VHF/FM band is very broad compared to the lower frequency medium wave broadcast band an IF frequency as low as 250KHZ can be used resulting in just resistance and capacitor coupled IF stages. This type of receiver also became a popular project in the late 1950s to late sixties as an easy way of receiving FM in HI FI quality Mono and Clive Sinclair also introduced the first FM match box portable set using this detection method, known as the Sinclair Micro FM using the newly introduced MAT Micro Alloy Transistors in which this design you are going to build is similar. The Pulse Counting FM Receiver is not without its disadvantages which I will briefly describe. Due to the low IF frequency used, In very good local reception areas the selectivity can suffer and to overcome this situation the more commercially made HI FI tuners used a double conversion approach consisting of a 10.7 MHZ first IF converted down to the low 250KHZ IF resulting in better image rejection. Also using this type of FM Detection does not go down very well with stereo reception unless the IF Stages have been designed with a high response curve such as not to have high value filter capacitors in the detector circuit then necessary that can lead to distortion of the 38KHZ Sub Carrier and S Signal components leading to poor stereo separation. Also because of the narrow bandwidth of this 250KHZ IF frequency, The 19KHZ pilot tone noise which we all describe as a constant hiss accompanied with background hetrodyne noise pulses is very noticeable and hard to suppress unless the signal is at least 60db minimum in strength. By referring to picture 6 which is the complete circuit diagram of the tuner and audio preamp section of this design I will give you a brief description of how this receiver works. The first stage consisting of the FET Transistor TR1 forms a wideband grounded gate RF Amplifier and its purpose is to isolate the antenna from the VHF Mixer/Oscillator which can cause radiation of the signal which may interfere with nearby listeners tuned to the same frequency. Also the purpose of the RF Stage prevents unreliable oscillation when certain antennas are connected due to the loading affect of the L1 tuned circuit. The second stage consisting of TR2 is the VHF mixer and oscillator and was originally the Superregenerative detector in our earlier design. I will describe how this is possible to make this stage perform as a frequency changer in this superhet configuration. Normally all superhet receivers need 2 RF coils for the oscillator and mixer to do both jobs. For example a typical FM portable receiver tuned to Radio 2 on 90.2 MHz has an IF Intermediate frequency of 10.7 MHZ. The mixer coil needs to be tuned to 90.2 MHZ to receive the signal. The local oscillator now has to be tuned to 79.5 MHZ below the received frequency which heterodynes with the original signal to makes up this sum and converts this signal to accept an IF frequency of 10.7 MHZ. This design is far less complicated in which I will tell you the simple reason. Because the pulse counting discriminator uses an IF Intermediate frequency of only 250KHZ, The frequency difference is so small that the regenerative detector used in the early receiver can form as an autodyne converter to do both jobs. The resistor R5 and capacitor C7 now form an IF filter to separate the high frequency components from being passed on into low frequency components which is the IF Stage described next. The next 3 stages which consist of TR3 TR4 and TR5 is a cascaded resistance coupled low Frequency IF Amplifier and these stages provide most of the gain by amplifying the converted IF signal. The 470PF coupling capacitors C9 C10 and C11 serve there purpose at setting the frequency response of each stage to form the low IF Frequency which is then coupled to the Limiter stage TR6. The purpose of the limiter stage is to remove interference pulses and shape the signal into a clipped waveform before signal detection can take place which is formed by the diodes D1 and D2 connected in a voltage doubler arrangement to form the diode pump circuit. The signal is then smoothed and filtered by resistors R16 R17 R18 and capacitors C17 C18 C19. Also this filter network form the de-emphasis components required when receiving a Mono signal. Just a brief note regarding C17 which is the diode pump reservoir capacitor. Its normal value 1000PF is ok if you just want to have a Mono receiver only. If you wish to experiment with a stereo decoder, You must decrease this value to 100PF to help recover the 38KHz Sub components and 19KHZ Pilot tone. TR7 is the AF Audio preamplifier and is the same circuit as the one used in the earlier Superregenerative receiver and is still needed to boast the signal to audio level so it will drive any power amplifier. On a closing note I wish you all the success in building these 2 simple VHF Receivers described and please refer to the following diagrams and information listed below if you want to proceed. A components list is also available at the following link Components List Of The Simple VHF Receiver Projects
Please left click on selected picture to enlarge image.
Please Refer To The Following Video Clips Below To See these Receivers Working
My Solid State FET Superregenerative VHF Receiver.MP4 - YouTube
My Demo Of Receiving High Quality FM Reception Using Straight Regeneration.MP4 - YouTube
My FM Superregenerative Receiver Converted To A Solidstate Pulse Counting Receiver.MP4 - YouTube
Construction Details Of The Simple 3 Transistor VHF Super Regenerative Receiver
Before commencing with the construction of these simple VHF Receivers it is advisable to check the component availability first by clicking on the following link Components List Of The Simple VHF Receiver Projects which gives you the full list of components and a list of well known internet component suppliers. Apart from the tuning capacitors and depending on what transistor is used for the TR2 VHF Detector all components should be available from the following link Maplin Electronics at this time of writing.
Important Guidelines Regarding Safety And Construction Layout Of This Simple VHF Receiver
Because VHF receivers use a much higher frequency then the Medium Wave and HF Shortwave bands it is not advisable for an absolute beginner to radio work to attempt the construction of this receiver unless they have had, At least experience with constructing the earlier featured regenerative receiver projects for the following reasons. The safety regarding the careful use of alkaline batteries and ensuring the correct polarity remains the same as for the earlier receivers except the construction of the RF circuits is very critical which I will briefly explain. Compared to the Medium and HF Shortwave bands every millimetre of lead counts as an inductance, So all wiring of the tuned circuits must be as short as possible and laminated copper cladboard forms an excellent ground plain in this respect and must be used for all the VHF circuitry. Normal veroboard can be used for all audio circuits as they do not carry RF currents in the same way as high frequency radio waves. Because this simple design is of the Superregenerative type of receiver, The risk of interference to other nearby FM receivers is very high and a metal case must be used for housing this receiver along with an RF Amplifier stage to isolate the antenna from the detector. As another precaution, Avoid using this receiver in the close proximity of airports particularly if you have it tuned to the aircraft navigation band as this could cause illegal interference to air traffic control. Also do not expect this type of receiver to give the same hi fi audio quality and sensitivity like my other receivers such as the 6 Valve VHF/FM Pulse Counting FM Tuner Using Safe 25Volt DC HT Line although having said that, If you happen to have a very good antenna system or live in a very good reception area this type of receiver can be used in normal regeneration mode using slope detection of the FM carrier and very excellent audio quality can be expected. Also if you are very lucky to live near a nearby strong signal transmission it is possible use my Special FM Stereo Decoder Circuit For Pulse Counting FM Receiver . I myself have experienced very good Hi Fi quality stereo reception when using the receiver in this mode and another advantage is because the detector is not oscillating you are not radiating illegal interference to nearby receivers. This receiver can also be modified to a slightly more advanced superhet design using the low frequency IF no coils pulse counting technology which will give even better performance regarding sound quality. One final note regarding the choice of FET Transistor used for the TR2 VHF Detector. The popular 2N3819 FET is the easiest transistor to obtain but its sensitivity is not as good as the BF244 particularly when using this receiver with a short telescopic antenna and stations are rather weak. The BF244B FET transistor is still available at the time of writing from the following link Cricklewood Electronics and is highly recommended particularly if you intend upgrading this version of the VHF Receiver to the later superhet pulse counting receiver.
General Construction Of The VHF Superregenerative Receiver
1. Before attempting construction of this simple VHF Receiver project it is advisable to have all the components needed at hand which are listed on the following link Components List Of The Simple VHF Receiver Projects .
2. Please refer to picture 2 for the complete wiring diagram of the basic receiver.
3. As you can see all the basic RF section is hard wired on copper cladboard and this includes the mounting of all the controls as well. Tagboard is also used for supporting the components and points marked E are chassis earth points.
4. The AF Audio section is wired on a veroboard which must be supported on the chassis with suitable bolts and is mounted on the top right side of the chassis with the volume and on/off switch above the circuit board as shown in the diagram. Also spacers must be used to isolate it from the chassis and PVC tape bonded around the board mounting area of the chassis is advisable. Also the complete audio board will serve for the later featured Pulse Counting Superhet Receiver so if you intend to do a later conversion, It will be advisable to leave plenty of space on the board for the IF Intermittent Frequency Amplifier.
5. The audio section must be wired up first as there is no way of knowing the receiver properly works without it unless you have a suitable audio source to hook the receiver up to. Please refer to picture 3 which is the complete wiring diagram of the entire audio section.
6. As you can see in the diagram of picture 3 the orientation of all semiconductors regarding correct polarity is explained, For instance the TR3 BC547 Preamp silicon transistor polarity is viewed from the top of the circuit board. The IC1 LM380N is also viewed from the top and a small half moon mould identifies its orientation. Also a 14 Pin DIL socket is recommended as this IC does not like too much prolonged heat when soldering. As like I have explained before in the earlier articles of the previous regenerative and crystal set circuits you must also pay attention to the correct polarity of the electrolytic capacitors making sure the positive + corresponds as shown in the circuit diagram. Picture 7 is the circuit diagram of the audio power amp circuit and I will just mention the purpose of the 4.7uh RF choke in the positive line of the speaker. It helps prevent VHF RF currents from circulating in to the output stage which is a known problem of instability which occurs when using short antennas and also when the volume is at a very high setting you may experience squealing which is described as VHF parasitic oscillation. This is a well known problem with a lot of the audio ICs particularly the LM386 used in my past FM receiver projects. Also all connection points on the circuit board are marked in alphabetical order to make interconnection to other stages easy and minimises mistakes. As I said earlier, It is advisable to use twin stranded screened audio cable for wiring the volume control and the braid must connect to the point G 0V line.
7. If you are confident you understand all the information explained above then you may go with the construction of this stage. Please refer to the information below for testing and setting up the audio stage.
Testing And Setting Up The Audio Stages
1. If you are double sure you have checked everything carefully such as the polarity of transistors, ICs and electrolytic capacitors you may now go ahead with testing this stage.
2. Connect a suitable 8 Ohm speaker to the output terminals H and J as shown in the picture 3 wiring diagram
3. Advance the RV2 preset potentiometer which regulates the supply voltage to the preamplifier and also acts as a gain control. Note that when this stage is connected to the receiver circuits this control may need readjustment which is with trial and error.
4. Advance the RV1 volume control about midway and taking care that you observe the correct polarity connect a suitable 9V power supply and switch on.
5. If all is well you should hear a soft hiss in the speaker. If this is so, Try touching point B with your finger or a screwdriver blade.
6. If all is well you should hear a buzzing noise which means this stage is working correctly and if this is so you may proceed with the wiring of the VHF RF Circuits which is the next procedure.
Wiring Up The VHF RF Circuits
This is where the greatest of care must be taken or the receiver will give very poor results or not work at all because as I said earlier every millimetre of RF leads to the tuning capacitor and RF Tuning coils counts as an inductance which results in high losses at VHF compared to the earlier shortwave receiver circuits. To give you an example please refer to picture 5 which is an inside view of my later pulse counting superhet conversion design, In which the same precautions apply. Also each VHF stage must carry its own earth return point to the chassis as shown in the wiring diagram of picture 2 or this again will stop the receiver working correctly as feedback from one RF stage to another is very noticeable in the VHF Ranges. A brief note concerning the TR2 Transistor which forms as the main detector in this VHF Receiver. The 2N3819 FET is satisfactory for tuning in very powerful nearby signals. However, You may find the receiver performs a bit on the weak side when listening to distant transmissions or when using the short telescopic antenna and for this reason I do strongly recommend you using the BF244B which is an excellent performer on the VHF Frequencies. Also the Biasing resistor R3 is different for both these transistors and is highlighted in the components list and the picture 1 circuit diagram. Also the transistor base views are viewed at the bottom for wiring this stage. Although a suitable heat shunt is advisable when soldering these transistors, It is the correct polarity that is the most important and you must double check this before applying power to the circuit.
Construction Of The RF Tuning Coils And RFC3 Choke
The L1 RF Tuning coil covers 87 MHZ to 120 MHZ when used with the specified 25PF air spaced tuning capacitor in the prototype receiver. This includes the VHF/FM Band and aircraft navigation band. Also this depends on the setting of the TC1 trimmer capacitor. It is also possible to pick up the Marine and Taxi transmissions on about 150 to 176 MHZ but as the local police are now using voice encryption techniques which were introduced in the early part of this century all these services are gradually going the same way and all you will hear is a unintelligible carrier which has took the fun out of scanning. The RF coil is airspaced and is made up from the solid copper strands of 6 mm UK Shower or Cocker type mains cable and about 6 to 7 turns is ideal for the coverage mentioned. A 1.5 Volt AA size battery is used for the assistance of winding this coil and its turns are spaced at about 1.5 mm. The RFC Regeneration choke is wound with about 25 to 40 turns of 26 SWG enamelled copper wire and the former can be a nylon rod of about 30 mm cut from a volume control potentiometer with thin layers of PVC tape at each end to support the windings.
The Final Wiring And Initial Testing Of The VHF Superregenerative Receiver
If you are now confident you have correctly wired up the VHF circuits as shown in the wiring diagram of picture 2 you may now connect up the audio stages ready for the initial test. Insulated tinned copper wire is used for all these connections and it is best to use red for all positive connections. Black wire is also used for all negative and earth return connections. Please follow the step by step instruction featured below.
1. Please refer to the picture 2 wiring diagram.
2. From the junction of R2 and R3 which is the positive supply line for the VHF RF circuits, Connect a red lead to point A on the audio board as shown in the wiring diagram.
3. Connect a Black lead from the C8 Audio coupling capacitor to the point B AF Input on the audio board as shown in the wiring diagram.
4. Connect a piece of bare tinned copper wire to the point C 0V Line to the point E earth point as shown in the wiring diagram.
5. Presuming you have already tested the audio stage it is now time for the initial test.
5. Reconnect a suitable 8 Ohm speaker to points H and I on the Audio Board which is the power amplifier output.
6. Connect about 1/2 a metre of insulated wire to the antenna socket or if you have already enclosed the receiver in the recommended case mentioned in the components list then fully extend the telescopic antenna.
7. Double check your work for any possible mistakes and if all is well reconnect the power supply circuit.
8. Rotate the RV1 Regeneration control clockwise until you hear a high pitched hiss which means the Superregenerative Detector is oscillating.
9. If this is the case then try rocking the VC2 tuning capacitor and it should be possible to tune in a few stations within the FM Band or the VHF Marine Aircraft band.
10. Try moving the antenna around for maximum signal strength until you get a clear undistorted signal with the absence of any background hiss. Also if the audio sounds a bit weak, Even with the volume near its full setting try readjusting the RV2 Gain control on the audio input preamplifier.
11. If all the above tests and procedures have gone ok it is now time to align the VHF tuning range so the correct coverage is obtained which I will briefly explain how this is achieved.
12. The frequency is at its most highest when the vanes of VC1 are fully open. Closing the vanes of VC2 Decreases the Frequency coverage of the receiver. The series trimmer capacitor TC1 in the tuned circuit also has the same affect and is used to set the frequency band limit of VC2.
13. In the case of us wanting the receiver to cover the entire FM broadcast band and aircraft navigation band you need to carefully follow these next steps.
14. Try to tune in BBC Radio2 which broadcasts between 88 to 91.0 MHZ which is extreme lower part of the FM band.
15. If you are receiving Radio2 with the VC1 Vanes half way open this needs correcting which I will describe in the next step.
16. Opening the Vanes of the TC1 highers the frequency coverage of VC1 and you need to slowly maintain this trimmer and at the same time and keep retuning radio 2 until you are receiving this station with the VC1 vanes open about 3mm from its lowest capacitance.
17. If all the above steps have gone to plan you now have a simple VHF receiver of all your own work. Please refer to these next important steps on how to get the best out of this receiver and simple tips on preventing interference to other listeners.
Simple Tips On Getting The Best Out Of this Receiver And Avoiding Interference With Nearby Listeners
Simple Superregenerative receivers can give the newcomer to VHF Construction, Many happy hours of fun but there is a disadvantage that this type of receiver can spoil a previous good friendly neighbourhood relationship for the following reason. Like I have said before Superregenerative detectors are always oscillating at a strong supersonic frequency with a quench frequency of about 30 KHZ. Unless the proper precautions such as enclosing this receiver in a proper metal case is not followed correctly it is possible to interfere with nearby listeners that are tuned to the same frequency. Even if you have took these precautions carefully it is also a good idea to confirm this by trying a domestic home portable FM receiver around different locations of the home to help confirm that you are not likely to course illegal interference. Even with the receiver in a fully enclosed metal case it is still possible to cause weak interference with a receiver that is in very close proximity so please still take extra precautions. As a final note, Do not let the cat out of the bag by asking your entire neighbourhood or showing them your piece of achievement as you may be automatically to blame for any future interference which is not necessarily your fault. If you are really concerned about causing interference then this receiver can also be used as normal regenerative detector in the same way as the earlier Shortwave regenerative receiver projects which I will describe how this is achieved.
How To Use This Receiver In Normal Straight Regeneration Mode
1. In the absence of a broadcast station advance the RV1 in the clockwise direction until the superregeneration hiss just begins.
2. Very slowly back off RV1 in the anticlockwise direction until the hiss just stops.
3. Try retuning in a strong FM signal which may sound very distorted and unintelligible because the detector is still oscillating in normal regeneration mode in the same way as the earlier medium and shortwave regenerative receivers. In the simple case of this you need to try backing off the RV1 Regeneration control slightly further and then it may be possible to tune in the signal crystal clear with no distortion. This method off receiving FM has its advantages and disadvantages which I will briefly describe. The main advantage is the detector is not oscillating at all so there is no chance of you causing illegal interference to nearby listeners. Also on very strong signals very high quality sound reproduction can be archived and it is even possible to use a stereo decoder due to the very high response curve. The main disadvantage is the receiver is not as easy to use practically for the none technical user for the following reasons. You need to keep readjusting the regeneration control when tuning from one station to another and you need to tune carefully on either side of the FM carrier for minimum audio distortion which is described as slope detection. If you wish to experiment with stereo reception then my Special FM Stereo Decoder Circuit For Pulse Counting FM Receiver works very well with this receiver. You will need to take off an audio feed point which can be obtained at point D and the earth can be obtained at point F which is at 0V negative potential which is where the outer braid of the connecting cable must branch off from. Please also note that stereo only works well on the very strong transmissions and an outdoor antenna is the best bet rather then relying on the simple telescopic antenna. As a closing note I hope you enjoy building and using this simple VHF Receiver project and it is convertible to a more powerful superhet broadcast FM Receiver using no coils technology which will follow next.
Construction Details Of The Simple FM Pulse Counting Superhet Receiver Conversion
Because this version of the FM receiver is a simple upgrade conversion from the simple regenerative receiver there will be little mentioned on the setting up procedure because if you have already built and got that design correctly working the mixer and oscillator circuits will be already aligned. If you are intending to upgrade the regenerative receiver you need to refer to picture 6 for the circuit diagram as there are 2 slight component changes required in the VHF RF Circuits and you must follow these step by step instructions very carefully.
1. Resistor value of R5 must be changed to 2K2.
2 Capacitor value of C7 must be changed to 1NF and C8 must be changed to 470PF.
3. The inclusion of the resistor RX may not be needed but may help to prevent the VHF mixer and oscillator breaking into superregeneration particularly when using BF244 as the mixer transistor or when new batteries have been installed. The value of this resistor is 1K.
4. As I assume you will already have the audio preamp and power amplifier constructed you only need the additional components listed below but please note the following. To avoid confusion of component values and duplicate numbering with the earlier design please make sure you are referring to picture 6 for the complete circuit diagram. You will also need to refer to picture 8 which is the wiring diagram for the superhet receiver configuration but please note the following. Because of drawing space I have compiled a separate wiring diagram for the IF and AF Circuitry in which you must refer to picture 9.
Additional components for the inclusion of an IF Amplifier stage needed for conversion
Resistors. All 1/4 cracked carbon
R6 R8 R10 R13 1M.
R7 R9 R11 33K.
R12 82K or 100K preset pot.
R14 R15 10K. R16 4K7. R17 R18 47K.
C9 C10 C11 470PF Ceramic disc.
C12 C14 100uf 25V Electrolytic.
C13 C15 100NF Ceramic disc.
C16 22PF Ceramic disc.
C17 1000PF Ceramic disc or 100PF if you want to experiment with a stereo decoder.
C18 100PF Ceramic disc.
C19 100NF Ceramic disc.
TR3 TR4 TR5 TR6 TR7 BC548
D1 D2 OA90 or OA91 Germanium Diode.
5. The above components list is for the additional components required to upgrade the existing Superregenerative receiver to a pulse counting superhet receiver version only.
6. If you are building this version of the receiver straight from scratch and have not yet built the earlier Superregenerative receiver then please refer to the Components List Of The Simple VHF Receiver Projects and look under the heading, Components List For The Simple Pulse Counting FM Receiver which lists all the components required to build the complete superhet receiver. Unless you are confident you can wire and assemble the whole superhet receiver from scratch it is advisable to build the Superregenerative receiver first and then follow steps 1 to 3 regarding the necessary component change.
7. If you are now confident that you can do this conversion you will need to temporarily disconnect the audio preamp and audio power amplifier board making sure you have disconnected the batteries before proceeding. To prevent any electrolytic capacitors on the board from recovering a charge it may on a temporary basis be advisable to connect a resistor of about 10K between the positive 9V line and negative 0V line. You must remember to remove this resistor when you come to reconnect everything or you may find yourself replacing new batteries frequently due to more excessive current demand then usual.
8. You now need to refer to picture 9 which is the wiring diagram of all the IF and AF circuitry.
9. Assuming you have left enough room on the circuit board after building all the audio stages, You may now proceed with building the IF circuitry. Please pay the usual attention like you did when building the earlier receivers regarding the component polarity of all semiconductors and try to keep soldering time to a minimum of about 3 seconds when soldering the D1 and D2 Germanium diodes as these are very heat sensitive.
10. Special note regarding the C17 Reservoir capacitor in the diode pump circuit and replacing R12 For a 100K preset pot. For normal mono reproduction this value should be 1000PF maximum. However, If you wish to experiment with a stereo decoder you need to make sure you do not increase this value to more then 100PF or stereo will not work or the separation in both channels will be very poor due to narrow bandwidth detection which will severely distort the 19KHZ pilot tone and S Signal components which includes the 38KHZ Sub carrier. Please note that this sort of receiver has its disadvantages when configuring for stereo which I will briefly explain. Due to the 250KHZ IF Intermediate frequency used, The bandwidth is much lower then receivers that have a 10.7 MHZ IF and the FM signal itself requires 150KHz for a distortion free signal. For the reason previously mentioned you need to be in a very strong signal area or have a decent outdoor antenna to give a signal of no less then 60 to 70db or results will be very poor and you will find background hiss and pilot tone noise very noticeable particularly on stations such as Classic FM and Radio 3 when there are quite intervals of soft music. Also if you are thinking of experimenting with stereo it may be a good idea to replace R12 with a preset pot or variable potentiometer as increasing the resistance will reduce the IF gain which will narrow the pulses in the diode pump circuit leading to wider detected bandwidth. Finally on this subject you will also need to take an audio feed from point D and point E which must connect to the outer braid of the lowest capacitance of screened cable you can possibly go for. A suitable stereo decoder circuit can be found at the following link Special FM Stereo Decoder Circuit For Pulse Counting FM Receiver .
11. If you are now confident you have rewired all the circuit board with the new IF circuitry it is now time to reinstall the the IF and AF circuit board ready for rewiring back to the VHF circuitry, Making sure you remove the temporary resistor link between the positive 9V line and 0V line on the circuit board.
12. The wiring to connections on the circuit board is carried out in alphabetical order in the same way as the Superregenerative receiver so mistakes are minimised and makes the wiring diagram more easy to understand.
13. If you are now confident you have completed all the above tasks very carefully it is now time to test and set up this version of the Pulse Counting FM Superhet Receiver. As a final reminder please double check the circuit board for any slight mistakes such as wrong component polarity and any possible solder bridges. please refer to the final testing and setting up procedure
Final Testing And Setting Up The FM Pulse Counting Receiver Configuration
1. Rotate the RV1 Regeneration control fully anticlockwise.
2. Rotate the RV1 Volume control between mid to high setting
3. Reconnect the batteries and switch on the power
4. If all is well you should hear a mid to high pitched hiss similar to the Superregenerative configuration which means the IF Amplifier and diode pump circuits are working correctly.
5. Rotate the RV1 Regeneration control very slowly until the high pitched hiss ceases which means the VHF Mixer and oscillator are working in Superregenerative mode overloading the IF amplifier stages which only needs some simple correcting which I will describe how to perform in the next step.
6. Slowly back off the regeneration control anticlockwise until you get to the point where the high pitched hiss recommences.
7. Connect a suitable antenna to the VHF antenna socket which can be a 1 metre piece of flex or the recommended telescopic antenna with its length fully extended. Make sure the antenna is placed in a vertical or horizontal position clear of any obstructions.
8. By rotating VC2 and assuming you have not done anything to upset the tuning alignment since having it configured as a regenerative receiver you should now be able to tune in the same stations as before but with nice crystal clear reception. You may due to increased sensitivity be able to hear stations that were previously very weak when you had the receiver configured as a simple regenerative receiver.
9. Adjusting the regeneration control is with trial and error but with a bit of practice your experience should gain. If you go for the strong stations on the FM band you will not need to hardly adjust it at all but you may find that the oscillation may cease when tuning from one end of the band to the other. Now that this receiver is a superhet configuration, This control is also now referred to as the oscillator amplitude control and having the oscillator set at its lowest amplitude as possible will reduce the likelihood of illegal radiation which is very low compared to the Superregenerative detector and also the 10.7 MHZ commercial standard broadcast receiver can experience similar problems.
On A Final Closing Note
I wish you all the success in building these simple VHF Receiver projects and that you also enjoy using them as well. This final solid state version of the FM pulse counting receiver is my best portable VHF receiver project to this very day and works very well in even the poorest of reception areas. This receiver also marks the final solid state project on this page.
Simple Valve Receiver Projects Designed For Safe Low Voltage Operation
Welcome to the last series of simple radio receiver projects on this page involving the use of valves, Our long and beloved devices. The first project is a simple low voltage regenerative receiver capable of receiving the Short and Medium Wavebands and will work on an HT voltage as low as 10 Volts. The second receiver project is almost the same design except it has a higher power output using an ECL82 output stage and will work on an HT Voltage of between 30 to 65 Volts. The third and final design is a VHF/FM version of the 2 valve regenerative receiver. Although these projects involve the safe use of batteries a mains power supply unit is featured for the more experienced constructors that know the dangers involving mains electricity and have at least built a mains powered project.
Please left click on selected picture to enlarge image.
Please refer to the following links below which are video clips of these receivers working
My New Design Of A 2 Valve Regenerative Receiver Working A Loudspeaker At 10 Volts HT.MP4 - YouTube
My Success With FM Stereo Using A 2 Valve Regenerative Receiver Driving An MC1310 IC Stereo Decoder - YouTube
Construction Details Of The Simple 2 Valve Low HT Regenerative Receiver
Before attempting the construction of these simple low HT valve projects please refer to the following link Components List For The Low HT 2 Valve Receivers and the important safety guidelines regarding the safe use of Acid Lead Batteries and the Low HT Mains Power Supply Unit.
Important Safety Guidelines Regarding The Safe Use Of Acid Lead Batteries And The Low HT Supply
Although these simple valve receiver projects use much lower HT voltages then the dangerous 250V DC supply found in mains powered receivers, The 6V heater supply involves the use of acid lead batteries which are not suitable for young children or inexperienced constructors to experiment with unless the following precautions are taken for obvious reasons which I will briefly mention. They are capable of delivering very heavy currents then the alkaline batteries used in the previous solid state regenerative receivers featured on this page in which you must observe the following precautions.
1. You must at all times be very careful not to accidently short circuit the terminals or this may produce large sparks and even rupture the cells which could lead to a fire hazard and the leakage of toxic acid.
2. You must also never wear jewellery such as metal watch straps when handling these batteries for the previous reasons mentioned and you must use the proper connecting crimps rather then just wrapping the wires loosely around the terminals as overheating may occur due to heavy current demand.
3. To ensure maximum protection against short circuits you must ensure that you use an inline fuse in the positive lead, Preferably in a plastic sealed enclosure before it enters the receiver which is the type used with car radios and is available from motor garage stores.
4. You must also make sure you use 6 Amp bell or stranded wire for wiring all valve heater connections and a suitable good quality switch must be used in the positive line capable of at least withstanding 3 Amp DC or contact welding may occur.
5. You must also make sure the recommended mains charger is used for charging these batteries and you must ensure the correct manufacture instructions are also followed.
6. If you feel intimidated about using acid lead batteries then 4 X 1.5V size C will work, Although you may find it expensive constantly replacing them unless you use the rechargeable type and you may only get an hour or so of listening.
7. The low voltage HT supply poses less of a problem apart from the higher powered version of this receiver that uses 54 Volts which I will briefly explain. Although 54 Volts is not dangerous like the common 250V supply found in mains powered valve receivers, Still do be careful as it is still possible to get an unpleasant nasty jolt. The same rules apply as mentioned in the solid state receiver article regarding the correct polarity of components such as electrolytic capacitors, But there is a good advantage about valves in this respect. As there electrodes are all of metal construction, Current only flows in 1 direction so it is impossible to damage these devices except the heaters may be blown if HT voltage is accidently applied to them, So still please do be careful and check your work constantly for possible mistakes. Also the ECC86 Triode must not exceed a maximum 30V of HT or melting of the anode may result.
8. Because valves use much lower HT currents then solid state devices you will find the batteries will last a very long time and PP3 transistor radio batteries connected in series will be suitable. Also because of the higher voltages incorporated with these simple valve receiver circuits it is still advisable to incorporate a fuse in the HT line for maximum protection against valve HT shorts although this is only common in guitar amplifiers which was mainly due to them being moved around from 1 venue to another.
9. Please refer to picture 5 for the recommended battery power supply circuit arrangement for powering these simple valve receiver projects. Pictures 6 and 7 are for a mains power supply unit that will in the long term reduce the cost of running these receivers on batteries but please refer to the following notes regarding safety. Although the LT and HT output is reasonably safe it still has 240V AC mains present on the primary side of the mains transformers and the circuit must only be attempted by experienced constructers who know the dangers of high voltages and have some experience with constructing mains powered projects. There will be construction details available for this circuit later on in this article.
10. As long as you fully understand these simple precautions you may now go ahead with the construction of these simple valve receiver projects which should provide you many hours of fun and over all, Great achievement in building your first valve radio.
Building The Receiver Of Your Choice
First time constructors to valve circuits and radio construction are best attempting the HF Short and Medium Wave version of these receivers first as the VHF/FM version requires care in the construction of the RF circuits regarding the layout and wiring. If you are building the first version of this receiver you need to refer to picture 1 for the circuit diagram and picture 2 for the wiring diagram. If you intend to build the higher powered audio ECL82 version you need to refer to picture 3 for the circuit diagram and picture 4 for the wiring diagram. Please refer to the next article which describes the chassis construction details.
Preparing And Constructing The Chassis
The hardest part of building any valve receiver is cutting and preparing the chassis. Alternatively if you wish to house this receiver in a wooden case you can use cladboard which also forms an excellent ground plain. Also Because of the low voltages involved you can use the old breadboard method of construction on both versions of the short and medium wave version. If you are intending to build the VHF version of this receiver I am afraid to say that a proper metal chassis or cladboard will only work as VHF circuits must relie on a solid ground plain with short point to point wiring because of the very high frequencies involved. It is best to have all chassis mounting components to hand such as controls, valve holders and tuning capacitors so you can get a rough estimation on hole sizes required before starting the drilling. The chassis punch tools for drilling holes to accommodate the valve holders will save you hours of filing but I am afraid to say that they are not as easy to get hold of like they once was when valves were a popular electronic device. A suitable chassis case is available from the following link Maplin Electronics and the order code is KR56L. Please refer to the following step by step procedures listed below.
1. If you have now confidant that you have all components available, The chassis drilling can now go ahead.
2. You need to refer to picture 2 the wiring diagram if building the basic low powered ECC86 amp version or picture 4 which is the wiring diagram for the higher powered ECL82 version.
3. The highlighted connection Points marked E are chassis negative earth return points and are usually a solder tag and bolt. You must ensure all these connections are screwed tight and are clean or sparking may result, Particularly within the valve heater connections as these are the most current hungry of the entire receiver. As the tuning capacitor and coax socket in the VHF design are of metal construction they will be automatically earthed to the chassis so it is just a matter of making sure the mounting bolts are screwed tight and the right connection tag is used for connecting the earth returns on the tuning coils
4. If all the above procedures have been correctly followed you may now go ahead with wiring and testing the valve heaters which will follow next.
Wiring And Testing The Valve Heaters
1. Although this is the easiest part of the wiring in the entire receiver you must ensure the correct size wire is used as mentioned earlier and all connections must be soldered as neat as possible with no dry joints.
2. As the chassis forms a negative earth return for the valve heaters you must make sure all solder tags are clean free of grease or sparking may result. Also the 100NF bypass capacitors must not be omitted or hum modulation may result particularly when using the mains power supply unit as the supply is 6.3V AC.
3. If you have followed the above procedures correctly you may now give this stage of the wiring a continuity and initial test.
4. You will need an analogue test meter set to the low ohms reading. If you are building the low powered version of this receiver you need to connect your probes in either polarity to SK5 and SK6. If you are building the high powered ECL82 version of the receiver you need to connect your probes to SK3 and SK4.
5. At this stage you should get a zero ohms reading. If for some reason you get a full reading at this stage then you have a fault and it must be investigated before proceeding any further.
6. Insert V1 and you should now get a reading of around 18 ohms. If this is so proceed to the next step.
7. Insert V2 and you should now find that the previous reading should drop to around 9 ohms. If this is so then this stage is wired and working correctly.
8. Carefully remove both valves and store them in there respective cartons until they are next needed. Proceed to the next step which is initial wiring and testing of this receiver.
Initial Wiring And Testing Of this Receiver
Assuming you have followed the last procedures regarding the wiring of the valve heaters and that the testing has gone ok you can now go ahead with the basic wiring of the receiver. As you are working at rather high frequencies try to keep all wiring in the RF tuned circuits as short as possible. When you come to initially test the receiver it is better to build it as a single band receiver and if your interest lies mainly in shortwave listening, The SW2 RF coil gives 24 hour coverage around all 4 corners of the globe and is also very easy to wind. You must pay very carful attention to the polarity of the electrolytic capacitors as these can heat up and explode as I mentioned earlier in the solid state article. If you are building the high powered ECL82 version then you need to ensure all electrolytic capacitors where highlighted are rated at 100V as a safety margin and that applies to all grid and anode bypass capacitors. You need to refer to picture 1 the circuit diagram and picture 2 the wiring diagram if you intend constructing the low powered 27V HT version of this receiver. Pictures 3 and 4 are the circuit and wiring diagrams if you intend building the 54V HT high powered version of this receiver. Picture 9 is the full coil winding details and you must ensure you use the correct gauge of wire as stated for maximum efficiency. The wire for winding all coils is available from the following link Maplin Electronics . Picture 8 is a simple band switching circuit for all the HF and Medium frequency range between 550 KHZ to 30 MHZ and it is best to test all coils individually before wiring it up. A 3 Pole 4 way switch is required and I must stress that you must try to keep all RF leads to the tuning capacitors as short as possible. As a precaution try to wire the switching arrangement in a way that the shortest leads are wired to the highest frequency coils rather then the reverse. 15 Millimetre plastic tube available from DIY Supermarkets is recommended for the coil formers and tie warps will hold the wire very secure as to using glue which can be very messy. Please refer to the last following procedure for testing and setting up this receiver.
1. Assuming you have wound the tuning coil of your choice and are certain that you have wired everything correctly you may give your newly constructed valve receiver a first time try which should be the moment of truth. As a final reminder please double check the wiring for any possible short circuits or any wrongly connected components such as the tuning capacitors.
2. Connect a suitable 8 Ohm speaker to SK7 and SK8 terminals.
3. Connect about 2 metres of wire to the SK1 antenna terminal for a temporary antenna for the time being.
4. Insert both valves into there respective holders.
5. Connect the green negative 6V heater supply lead to SK6 and the other end to the negative terminal of the acid lead battery taking the greatest of care not to short the supply. NB This terminal is SK3 on the 54V ECL82 version of this receiver.
6. Connect the yellow positive heater supply lead making sure this side of the supply is correctly fused with the correct 2 amp fuse as stated to the SK5 terminal. Connect the other end to the positive terminal of the acid lead battery. NB This terminal is SK4 on the 54V ECL82 version of this receiver.
7. Switch the heater supply on and if all is well the valve heaters should start to glow red which means this side of things is ok.
8. Switch off the valve heater supply for the time being until the HT side of things is fully connected.
9. Advance the RV1 regeneration control about midway and the RV2 volume control fully clockwise.
10. Connect the black negative HT lead to the SK3 0V terminal. Connect the other end to the negative terminal on the 27V HT battery. NB This terminal is SK6 on the 54V ECL82 version of this receiver.
11. Connect the red positive HT lead to the SK4 HT positive terminal. Connect the other end to the positive terminal of the 27V HT battery making sure the correct 100 milliamp HT fuse is connected in series and a suitable rated on/off switch is used. NB This terminal is SK6 on the 54V ECL82 version of this receiver. Stand by for the moment of truth.
12. If everything has gone to plan your valve receiver should now be working and by rocking the VC1 tuning capacitor you should be able to tune in a few stations
13. Try rotating the RV1 regeneration control until the receiver starts to oscillate, that can be identified by a continues beat noise.
14. Backing off RV1 until the oscillation stops is the most sensitive working limit of the regenerative detector and stations should be received at very good signal strength depending on the time of day when listening on shortwave.
15. If you are finding it impossible to obtain oscillation at the lower end of the frequency range you will need to add 1 or more turns on the earthy end of the L1 tuning coil. On the other hand, If oscillation is uncontrollable you need to take a turn or more off the earthy end of the L1 tuning coil.
16. If your interest is listening to amateur radio hams then it is possible to resolve SSB Single Sideband. 3.5 MHZ 80 metres is best during the evening hours and 14 MHZ 20 Metres is best during the daylight hours. Strong sideband signals are identified in an unintelligent distorted speech when the regenerative detector is not oscillating but can be resolved by rotating the RV1 regeneration control clockwise until the set goes smoothly into oscillation. You may then have to tune through the signal very careful and compared to normal AM reception SSB stations sound more like Donald Duck or if you are talking in a tunnel.
17. This now completes this article regarding the simple 2 valve regenerative receiver project and I wish you all the success in getting your first valve receiver working and many hours of listening enjoyment. If your interest lies within listening to VHF/FM then the next article features a 2 valve regenerative version which is constructed in slightly different way to this version regarding the RF circuits. To close this article on low voltage valve AM and Shortwave Receivers why not visit this link 12V Superheterodyne Receiver designed by my favourite radio enthusiast John Hunter. His excellent website has only just been redesigned at the time of writing and I have only just learned he has just included a 12V VHF/FM Super Regenerative Receiver similar to the design that follows and can also be found at the following link 12v B+ Super Regenerative Receiver
2 Valve VHF/FM Regenerative Receiver
This simple 2 valve VHF regenerative receiver is constructed in the same way as the previous valve receiver projects and as the safety precautions regarding the safe use of the Acid lead Batteries remain the same, Little will be mentioned on that side of things except that this circuit uses very high frequencies and all RF circuits regarding coils and circuit layout must be arranged in a way that leads to the tuning capacitors must be as short as possible. Band switching and using the long wire medium and shortwave techniques will result in the receiver giving very poor performance and in some cases not working at all. Also do not expect to get the same performance regarding sensitivity like you would when using receivers such as my 6 Valve VHF/FM Pulse Counting FM Tuner Using Safe 25Volt DC HT Line unless you have a suitable outdoor antenna designed for receiving VHF/FM transmissions. In very good reception areas it is possible to use a short 1 metre piece of wire or better still, A telescopic antenna. Another closing note regarding the regenerative detector configuration. The regenerative detector works in the same way as in the previous valve receivers except positive grid bias is obtained from the anode rather then leaky grid bias through the R5 270K resistor. Increasing R5 to around 470K or more will make the sett operate in Superregenerative mode except I found performance to be very poor regarding sensitivity and sound quality was nothing to be desired compared to using the straight regenerative technique. Before attempting the construction of this receiver please refer to the safety guidelines mentioned in the earlier valve receiver article and the Components List For The Low HT 2 Valve Receivers . You also need to refer to picture 10 for the circuit diagram and picture 11 for the wiring diagram. Please refer to the next procedure which is the construction details of the VHF RF Coils.
Construction Details Of The VHF RF Coils
The RF coils used in this design are self supporting and ether 18 SWG tinned copper wire can be used or the bare strands of 30 Amp cooker mains cable. A spare Size AA penlite battery will assist in winding the coil. L1 consists of about 7 turns and is slightly spaced at about 2 millimetres between the windings. The L1 antenna tap is taken about 2 turns from the hot end of this coil which ever gives best results. L2 is wound in the same way and consists of about 7 turns. The cathode feedback tap for L2 is taken about 2.5 turns from the earthy end of the coil as shown in the diagram. Assuming you have now completed all the above procedures carefully and checked your work for any wiring errors such as ensuring the correct polarity of the electrolytic capacitors, You may now go to the next and final procedure which is the testing and setting up of this VHF Receiver project.
Testing And Setting Up
1. Connect a suitable VHF antenna to the coaxial antenna socket and insert the 2 valves into there respective holders.
2. Advance the RV1 regeneration control fully clockwise.
3. Advance the RV2 volume control fully clockwise at maximum volume.
4. Connect a suitable 8 Ohm speaker to terminals SK5 and SK6 which is the speaker output.
5. Connect the green negative 6V heater supply lead to the SK1 0V heater terminal. Connect the other end to the negative terminal of the 6V battery.
6. Connect the yellow positive 6V heater supply lead to the SK2 positive heater terminal. Connect the other end to the positive terminal of the 6V battery making sure this side of the supply is well fused with the recommended 2 amp fuse and has the correct rated switch.
7. The valve heaters should start to glow red within around 10 seconds which means this side of things is working ok. If this is the case, You can now turn off the valve heater supply for the time being until the 27V HT side of things is connected up which should be the moment of truth.
8. Before proceeding with the 27V HT connections it is advisable to double check your work for any possible mistakes such as wrongly connected electrolytic capacitors and short circuits.
10. Connect the black HT negative lead to the SK4 0V terminal. Connect the other end to the negative terminal of the 27V HT battery.
11. Connect the red HT positive lead to the SK3 HT positive terminal. Connect the other end to the positive terminal of the 27V HT battery making sure this side is fused correctly with the correct rated 100 milliamp fuse and has a suitable rated switch incorporated.
12. Switch the 6V heater and 27 Volt HT supply on.
13. Allow 1 minute or so for the valves to warm up
14. Try rocking the VC1 tuning capacitor and you should be able to tune in at least 1 of the stronger FM stations but with distorted intelligibly until the next following step has been corrected.
15. Slowly rotate the regeneration control anticlockwise to the point where the receiver stops oscillating and it may be possible to tune in the station very clearly. Note that because you are using slope detection it will take a bit of time to get used to tuning this receiver correctly as you have to accurately tune to ether side of the FM carrier. Also like the short and medium wave receiver projects the regeneration control needs to be readjusted each time the receiver is retuned to another station. Now you have got this FM receiver project working ok please go to the final part of the setting up which is the RF tuning alignment.
RF Tuning Alignment
If you have built the earlier solid state FM receiver projects the tuning alignment is almost the same but as this version may be a first time FM receiver project for some constructors I will do a brief rundown on the final stage.
1. Tune in a station around the mid part of the FM band which is around 96 MHZ.
2. Adjust TC1 the antenna trimmer for maximum signal strength.
3. Try tuning in BBC Radio 2 which is at the lower end of the FM Band between 88 to 90.2 MHZ.
4. TC2 which is connected in series with the VC1 tuning capacitor is used to correct the FM tuning range which I will briefly explain. Opening the vanes of TC2 swings the FM coverage in the high frequency direction. Closing the vanes of TC2 swings the FM coverage in the low frequency direction. If you find for example that you are receiving BBC Radio 2 with the vanes of VC1 halfway open then this is very easy to correct. Readjust TC2 by opening the vanes very slowly. You then need to retune to Radio 2 and keep maintaining this procedure until you can receive this station with the VC1 vanes open around 5 millimetres from the closing point of VC1. As the FM band is very broad compared to the low frequency medium waveband, TC1 should not need realigning all that much but it may worth retuning to some of the weaker stations around the middle of the FM band and readjusting this trimmer for maximum signal strength
5. If all has gone well with the previous alignment steps, Then the receiver is ready to use and I wish you all the best of listening enjoyment out of this simple valve FM receiver project. Before I close this series on the simple valve FM Receiver project please refer to the simple procedures below if you intend connecting this receiver to an external amplifier or using a stereo decoder.
Connecting This Simple FM Receiver To An External Amplifier or Stereo Decoder
This simple FM receiver is capable of giving very high quality sound when connected to an external HI FI amplifier and if the signal strength is very good, A stereo decoder will also work. Just 2 simple precautions regarding safety before proceeding. Do not attempt to connect to any valve amplifier or radio that incorporates a live chassis or a serious electric shock hazard will exist. The way of finding this out is to look on the back of the set and if it says universal AC/DC mains then it certainly is unsafe to use. Also the UL84 and UY85 100 Milliamp heater type valves will exist that obviously gives evidence. Also a 0.1uf coupling capacitor of at least 500V rating must be used. Please refer to the simple procedure below.
1. Connect a 0.1uf 500 or 1000V polypropylene capacitor to the pin 1 anode pin of V2 for an audio take off point. Connect the centre strand of a suitable screened audio cable to the other end of the capacitor. Connect the outer braid of the audio cable to the 0V line. It may also be better in the long run to incorporate an RCA Phono socket on the back of the receiver for this purpose. You will have to mute the internal speaker by using a switching network to an 8 Ohm 1 watt resistor but as the audio output is so low there is very little risk of saturating the core of T1 unless a higher powered amplifier is used for the output stage. When connecting the receiver to a stereo decoder you must make sure that the screened cable capacitance is not to high or very poor stereo separation will result and you must make sure that this lead is as short as possible preferably not more then quarter of a metre between the decoder and receiver. A suitable decoder that works can be found at the following link FM Stereo Decoder Circuit and also to give you an idea of how this same receiver performs in stereo please refer to the following link to see this video My Success With FM Stereo Using A 2 Valve Regenerative Receiver Driving An MC1310 IC Stereo Decoder - YouTube .
Suitable AC Mains Power Supply Unit For Powering All The Valve Receiver Projects
This final project on this page is an AC mains power supply unit suitable for powering all your valve receiver projects on this page and in the long run, It will eliminate the use of batteries particularly when using your valve receiver indoors saving you the cost of constantly replacing them and the time it takes recharging the 6V heater battery. This simple power supply is very easy to build but please note the following points before attempting the construction. You must remember that high voltages can kill and because this project involves the use of 240V AC mains electricity it is not suitable for anyone who has had no experience with building mains powered projects or does not fully understand the dangers that high voltages can pose. As a final reminder, If you think you are unconfident with building this project then at least stick to using batteries or get someone who is experienced to build and test it for you. Please remember that you are taking your own risk and although you may get this power supply working ok this does not necessarily mean to say that it will be safe to use unless the following precautions are taken which are listed below.
1. It must be built in a metal case designed for this purpose with suitable ventilation and the vent slots must be small enough so that small fingers can not access any live parts or connections.
2. The metal case and 0V negative line must be soundly earthed by the green/yellow mains core and the earthing bolt on both mains transformers must be very securely tight. Also make sure the solder tags are clean with no signs of grease or dry joints or the earth will not be very effective. Also the mains plug must be fused at 3 or 5 Amps maximum and must be a proper 3 pin non reversible type.
3. You must also use use a proper rubber grommet for entry of the mains cable rather then the simple tying a knot idea which used to exist in projects featured in radio magazines during the valve era of the 1950s which I am afraid to say that electrical safety rules were not as strict in those days as they are now.
4. Also to protect the power supply unit from current overloads or the possible risk of fire you must ensure that the mains and HT fuses are rated correctly. Do not cut the corners by using old tricks such as cigarette paper or any foreign object or you may burn out your mains transformers and there could be a possible risk of fire. The 250 milliamp mains fuse FS1 which is in the live side of the mains protects the primary side of the transformer against any possible overload or fault. The 100 milliamp HT Fuse FS2 in the secondary HT output protects the bridge rectifier and mains transformer, So any heavy overload or short circuit will blow this fuse immediately. If this fuse keeps frequently blowing there is a fault and it must be investigated before proceeding any further.
5. If all these precautions are followed correctly then this power supply circuit will be absolutely safe to use as batteries, Provided it is not exposed to wet or damp conditions.
6. If you feel confidant about building this circuit then you may now refer to the following construction and testing procedures listed below but if you do feel in doubt, Stop now as you are taking your own risk.
Constructing The AC Mains Power Supply Unit
1. Please refer to picture 6 for the circuit diagram and picture 7 for the wiring diagram.
2. Points marked E in the wiring diagram are chassis earth points and you must make sure that the ones that accommodate both mains transformer bolts have a solid bond to the mains earths green and yellow core. Also you must ensure that that the HT 0V Line and 0V LT Heater Line is also earthed to this point as shown in the circuit and wiring diagram. If this step is followed correctly there should be no danger of the case and your radio receiver project becoming live at mains potential, In the case of a serious fault within the mains transformers.
3. It is better to have all chassis mounting components to hand before starting the construction so you can get familiarise yourself with a good estimation of the hole sizes that need to be drilled in the chassis. Also it is better to have all these components mounted on the case before proceeding with the wiring. The bridge rectifier and HT smoothing circuit, That is C1, C2 and the R1 smoothing resistor are mounted on plain veroboard. As a final reminder you must make sure that the electrolytic capacitors C1 and C2 are connected in the correct polarity or these may explode and result in dangerous circumstances. Although these low powered valve project consume little HT current it is also a good idea to make sure that there is at least 2 millimetres of space between R1 and the circuit board.
3. If you are using a 54V HT line to power the ECL82 version of the valve receiver you must ensure the electrolytic capacitors are rated at least 100V as a safety precaution and that the 2 20V AC secondary windings on the HT mains transformer output are wired in series.
4. If you are using a 27V HT line to power the ECC86 and VHF/FM version of this receiver then 63V electrolytic capacitors can be used. You also need to ensure that the 2 20V AC secondary windings on the HT mains transformer are wired in parallel.
5. If you are confidently sure you have followed all the above procedures correctly you may now go ahead with the testing and setting up this circuit but as a final reminder. Do double check your work and ensure that the greatest of care is exercised when testing this circuit. Remember in the interests of safety that if you have any doubt, Do get a least a qualified electronic engineer to test it for you.
Testing And Setting Up The AC Power Supply Circuit
1. Insert the 250 milliamp mains fuse into its respective holder but at this point do not plug the unit into the mains until instructed to do so. Also do not insert the HT output fuse at this point. To ensure that this unit is safe to use and ensuring nothing dramatic will happen when first switching on, A continuity test is to be first carried out. Please go through the following procedures very carefully.
2. Set your analogue test meter to the low ohms setting.
3. You need to make sure that at least one of your test probes preferably the black lead has a crocodile clip for this exercise
4. Strap the probe with the crocodile clip to the neutral prong of the the mains plug marked N.
5. Using your red test probe, Touch the live prong of the mains plug marked L.
6. If the power supply unit is switched on then you should obtain a reading of at least a few hundred ohms. At this point try rocking the mains on/off switch to see if this is working properly. If this is so then this test is OK so far and you can proceed to the next step.
7. Strap the probe with the crocodile clip to the earth prong of the mains plug.
8. Make sure that the unit is still switched on.
9. Using your red test probe, Touch the neutral prong of the mains plug marked N. If you do not get any reading, Not even a slight kick of the needle then repeat the same step but instead touch the live prong of the mains plug marked L. If you get the same results then this step has gone ok and you can next proceed to the final step of the mains wiring continuity test.
10. Leave the black crocodile test probe strapped to the earth prong of the mains plug marked E. Try touching the the metal case with your red test probe. At this stage you should get a zero ohms reading which means a full kick of the needle. Repeat the same step but at this time touch your red test probe on the SK2 0V negative 6.3V heater output terminal and the SK3 0V HT output terminal. If you get the same results then the AC mains wiring and earth continuity test is fine and this mains power supply unit will be absolutely safe to use provided no exposed live connections can not be accessed as stated earlier. Please refer to the next step which is a simple continuity test of the HT rectifier and smoothing circuit.
11. Set your analogue test meter to the low ohms reading. In ether polarity touch both test probes on the bridge rectifier input terminals. If all is ok the needle on your test meter will kick to a zero ohms reading and then go back down to a high resistance reading due to the charging reaction in the electrolytic capacitors. If you get a solid full reading or nothing at all then you have a fault and it must be investigated immediately before this unit can be connected to the mains.
12. Insert the 100 milliamp HT fuse.
13. Set your test meter to the DC Volts range of around 100 to 200V
14. Because you are applying AC mains for the first time, You need to have a mains socket arrangement so that you can stand at least 2 metres away from the unit in case something dramatic happens such as a wrongly connected smoothing capacitor exploding.
15. Observing the correct polarity, Connect your test probes to the SK3 and SK4 HT output terminals.
16. At this moment do not plug into the mains.
17. Make sure the unit is switched on at the on/off switch.
18. Plug the unit in and step well back for at least 15 seconds.
19. If all is well you should get a meter reading of at least 28 to 35 volts and if using the higher HT voltage this should be around 55 to 70 volts.
20. If you hear a loud hum or smell burning then you must switch the unit off immediately and recheck your work for any possible faults.
21. Switch the unit off for the time being and remove your test probes from the HT output.
22. Reset your test meter to the AC Volts range of 20V.
23. In ether polarity, Connect your test probes to the 6.3V Heater output terminals SK1 and SK2.
24. Reconnect the power and you should now get a meter reading of a around 6.5 to 7V. If this is the case then you have successfully constructed this power supply unit and it can now be used safely to power your valve receiver project.
25. It is connected to the receiver in the same way as the battery supply but just a few closing notes. The same gauge wire is used for all HT and heater connections but as the power supply is already fuse protected the inline fuses for the battery supply are not needed but can be retained in case you still decide to use your receiver away from a mains electricity supply. As a final precaution do make sure this unit is disconnected from the mains before attempting to connect your valve receiver projects. Also when this unit has been switched off it is normal for the electrolytic capacitors to still hold a charge and in this case it may be worth connecting a 1 Watt 470K resistor across the HT terminals to gradually discharge this voltage as a wise precaution before attempting to connect your valve receiver projects.
I am very pleased to say that after 1 year of extensive on and off work, This page related to the construction of simple radio receivers is now complete and I wish you all the pleasure of reading this page and hope new and old constructers enjoy experimenting with these simple radio receiver projects. Have Fun.
Site Map Of All My Webpages And Favourite Valve Radio Related Links