6 Valve VHF/FM Pulse Counting Tuner Using Safe 25 Volt DC HT Line
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This part of my site features a valve version of the VHF/FM Pulse Counting FM Superhet Receiver. It works very much in the same way as my Transistor FM Superhet Receiver but uses valves and the unusual thing about this design it will work on low voltages even as low as 15 volt if the local oscillator anode decoupling resistor is optimized to a lower value to suit. Low voltage operation of valve radio circuits is not really anything new and circuits for 1or 2 valve regenerative short wave receivers have appeared in radio magazines of that era. Also car radios of the late 50s used this method using special space charge valves designed for 12 Volt operation for the radio section and the usual AD161 and AD162 germanium power transistors used for the power output stage to drive a loudspeaker. It was still necessary in some cases to use valves particularly if it was a portable FM set as transistors suitable for VHF work had not quite caught up in technology until the early 1960s. Although RF and IF amplification is possible using normal mains valves at these low voltages, the power output is very limited because the emission of the cathode current is only about 1 to 3 milliamps at these voltages. However. If you are in a quite listening environment such as a bedroom or small office you can get usable results of about not more then 3 miliwatts of audio power from a small EL84 power valve or an even more economical valve such as the ECC86 designed for low voltage operation. The ECC86 has a cathode current of 10 milliamps per triode and will give a loudspeaker output of about 100 milliwatts from the main 25 Volt HT line at very pleasing volume and good audio reproduction. It also takes less heater current then the other power valves. The pulse counting superhet receiver became a popular circuit for amateur Hi Fi experimenters during the mid 1960s to early seventies as a low cost means of receiving FM in Hi Fi quality Mono and the advantages of it is there are no 10.7 MHZ IF transformers involved or hours of RF alignment of Ratio Detectors to get a design that is distortion free working almost first time when powered up. The disadvantages of the pulse counting receiver is the low IF bandwidth available for reliable multiplex stereo transmissions for feeding a stereo decoder along with the 19KHZ pilot tone and 38KHZ subcarrier. Although it is possible with this design the separation is not as good compared to the 10.7 MHz Ratio detector and needs a far more stronger signal then mono of around 60db for hiss free reception. I will describe how mine valve version of the pulse counting FM receiver works as follows. This design uses 5 double triode ECC88 valves for the RF Amplifier, Mixer and IF Amplifier and although they have become a popular audio valve for preamplifier applications it was originally designed as a framed grid VHF Valve for use as a cascode VHF RF amplifier and self oscillating mixer in Television and FM Receivers. I had a good collection of ECC88s and found out by experimenting with a 1 valve VHF Regenerative Receiver that they work very well with low anode voltages of around 7 volts. Referring to the block diagram and circuit diagram of picture 5, V1A functions as a wide band grounded grid RF amplifier and although it does not really provide very high gain its purpose is to isolate the VHF Antenna from V2B that would otherwise course detuning effects resulting in unreliable oscillation when tuning certain frequencies of the FM band and most importantly of all, oscillator radiation that can effect the reception of nearby receivers. V2B is an electron coupled self oscillating Autodyne mixer and although its appearance is very similar as the regenerative detector used in the Simple 2 Valve TRF Tuned Radio Frequency Receiver it can perform both functions as an oscillator and frequency converter with 1 RF tuning coil. The reason for 1 coil performing both tasks is because the 250KHZ IF frequency separation is so small that there is hardly no loss in RF gain and with a normal 15PF tuning capacitor it is possible to tune the entire FM band between 87.5 MHZ to 108 MHZ. This section alone can also function as a 1 valve regenerative receiver which was an experiment that led to this design. The 1000PF decoupling capacitor C7, function along with the C15 grid coupling capacitor for V2A to form an IF filter to reject the VHF megacycle frequencies and only amplify the wanted low frequency of 250KHZ through the IF stages. V2A to V5A function as a 7 stage 250KHZ Intermittent Frequency Amplifier. The 470PF grid coupling capacitors C9 C10 C14 C15 C18 and C20 set the IF frequency response to around 250KHZ. V5B functions as a limiter by applying a slightly lower plate bias to this stage and the clipped wave form is then applied to the demodulator stage. The V6 6AL5(EB91)Valve functions as a voltage doubler diode pump charge FM Demodulator and the resistance capacitive filter R25 R26 R27 along with C26 C27 and C30 form a simple deemphasises filter to reduce background hiss and also sets the frequency response of the transmitted audio to the UK time constant of 50 microseconds before being amplified to an audio signal. Although this tuner was originally planned as a 6 valve design the V7A ECC82 half triode section functions as a AF voltage preamplifier to amplify the audio signal to a suitable level to drive any valve or transistor amplifier. V7B is not needed unless you decide to add a power amplifier stage into this tuner and to avoid cathode poisoning its heater must not be connected unless the reason to use this section is required. This tuner if built properly performs just as well and in some cases better then FM tuners that need a higher voltage HT line. It has also been designed with safety in mind using a low 25 volt HT line and providing care in the construction of the recommended power supply unit in picture 3 is taken, this receiver should provide many happy hours of pleasure. Please click on the following link YouTube - Phil's design of a valve FM pulse counting tuner using safe 25 volt HT line to hear this design working and refer to the guidelines and diagrams below for construction details. Have Fun.
Block Diagram Of The 6 Valve VHF/FM Pulse Counting Receiver
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Construction Details Of This FM Tuner
Step 1. Important points to know regarding safety and general construction tips
Although this is the safest valve project to build on this site using a safe 25 volt HT line there are several safety precautions that need to be born in mind as you are still dealing with 240 volts AC mains.
1. You must only use the recommended power supply circuit as described in pictures 3 and 4 and all mains connections must be covered using proper insulated materials and an earthed metal case is essential with a screw fitted lid so no dangerous live parts are accessible.
2. You must use the correct rating of fuses for the power supply circuit. The FS1 1 Amp mains fuse only protects the mains side of the power supply in the case of a faulty switch, wiring or a fault in the primary of the mains transformers. The FS2 100ma HT fuse in series with the bridge rectifier protects all the low voltage circuitry and T2 Secondary from overloads. This fuse will blow immediately if there is any serious fault such as a short circuit or high current overload on the HT output. Do not for any reason increase this fuse higher or try the old tricks such as tinfoil or cigarette paper to stop this fuse from repeatedly blowing or you may burn out your mains transformers and there could be a big risk of fire.
3. You must also take care regarding the polarity of the electrolytic capacitors. Always make sure that the positive plus + side corresponds as shown in the circuit or wiring diagram and it is recommended that these capacitors are rated at no less then 35 volts for this circuit. Incorrect polarity of the electrolytic capacitors can result in them exploding and can also be dangerous.
4. The valve heater wiring regarding the current rating along with the heater transformer must be taken into serious consideration as the heaters are the most current hungry of the entire tuner. The 6.3V valve heater circuit with all 7 valves in circuit consumes 2.35 Amps. Although a mains transformer that provides 6 volts at 2.5 Amp would be the nearest safe option I would recommend around 3.5 to 4 Amps in case you might decide to upgrade it with a power amplifier using the popular EL84 or ECL82 output valves. Also you must use the proper 6 Amp stranded or bell wire twisted to cancel mains hum from the outgoing connection from the power supply to the tuner. The aluminium chassis acts as an earth return for all the heaters in circuit so you must also make sure all solder tags and bolts are securely tight or sparking and crackling noises will result, spoiling the good performance of this tuner. The 100N ceramic disk capacitors that are parallel connected across the heaters of V1 to V7 that is C4 C11 C13 C16 C21 C24 C33 are RF bypass capacitors to help prevent feedback of VHF oscillations flowing though the heater circuit and must not be omitted.
5. It is important that this tuner is built in a diacast or aluminium chassis to prevent radiation of the local oscillator from effecting the reception of nearby FM receivers tuned to the same reception frequency. It is also advisable to make sure you have a bottom lid on the chassis to screen the VHF circuitry. The use of a wooden chassis to make the receiver look like the 20s and 30s style of design is not recommended for the same reason I have just mentioned. Also VHF designs, weather regenerative or of the superhet design do need a decent ground plain to work really well regarding stability.
5. Important points to remember about connecting to other audio amplifiers. The ECC82 Line driver stage will feed any IC audio amplifier of the LM386 and TDA7052 variety without any problems and should be able to drive almost any transistor amplifier. Connecting to other valve amplifiers should also pose no problem providing the following points are born in mind. First of all you must make sure the anode coupling capacitor C34 of the audio driver stage is rated at least about 400-1000 volts as a high voltage fault on the input in which you are connecting to may damage components and could pose an electric shock hazard. Some of the older valve amplifiers such as the Quad and Mullard 5-10 variety that were around in the 50s have HT and LT sockets for powering ancillary equipment such as preamplifiers and tuners. Again this voltage is far to high for feeding this tuner and because the heaters may be centre tapped at 3.0.3 volt to earth, this sort of supply would be very unsuitable. Finally on a closing point regarding safety there is one kind of valve amplifier or radio that this tuner must on no account be ever connected to unless a suitable earthed mains isolation transformer is used. It is of the AC/DC variety that uses a live chassis and does not incorporate a mains transformer. A big green dropper resistor usually exists along with the UCL82 and UY85 type valves with there 100ma heaters. The Mullard 4-7 push pull amplifier falls into this category and could pose a very lethal electric shock hazard. However, with a suitable earthed isolation transformer these problems can be safely overcome.
Step 2. Component Availability
There is one good advantage when building valve equipment that uses a low voltage HT line. The smoothing electrolytic capacitors that are used in the power supply and receiver itself are more easier to obtain along with other components because of there lower voltage rating required. Resistors apart from in some rare cases, need only be 1/4 of a watt because of the low power and heat consumption involved. The only really hard items to obtain for this design are the tuning capacitors, through not impossible there could be a lengthy wait depending on the response of the delivery concerning the order of these hard to get components, due to excessive demand. It is also possible to use old 365PF tuning capacitors found in medium and short wave receivers if a series 10-30PF trimmer is used to reduce the capacitance, although there is a high risk of electrical backlash involved such as scratchy tuning particularly if the vanes are shorting together or have been damaged due to corrosion. Please click on the following link Components List Of 6 Valve VHF/FM Tuner for full information regarding the components for this tuner and lists of suppliers
Step 3. Wiring And Testing The Power Supply
This is where the greatest of care must be taken as this involves 240 Volts AC Mains and any slight error regarding the wiring of the mains connections, particularly the earthing could lead to a serious risk of electric shock. If you are unsure about how to proceed with the wiring and testing of this unit then at least get an electrician to check the mains wiring before plugging it in and switching on.
1. If you have got all components to hand then please refer to picture 3 for the circuit diagram and picture 4 for the wiring diagram.
2. You need a multimeter capable of being able to read at least 500 Volts AC and 500V DC
3. It is best to get all components such as the mains transformers, switches and fuse holders mounted into the case first before wiring everything up and mounting the circuit board.
4. Start by doing the mains wiring first, making sure all solder joints are neat and tidy with no dry joints.
5. Make sure the green and yellow core of the mains cable is securely tight on both earthing points of the mains transformers as shown in the wiring diagram.
6. If you are sure all mains wiring is correct you can actually test this side of things without plugging the unit in. It is called a continuity test which is also carried out on all house electrical installations before connecting and switching on.
7. Inset the mains lead including FS1 but do not on any account attempt to plug in and switch on
8. Set your multimeter to the 500 to 1000 Ohms range.
9. By touching your test probes on the live and neutral pins of the plug you should get a reading of around 50 to 500 Ohms depending on the resistance of the mains transformers. If this reading is zero Ohms which means a full reading when using an analogue meter then something is seriously amiss such as a short circuit somewhere in the wiring and must be investigated.
10. Try rocking SW1, the mains switch on and off to check this also works correctly.
11. If all the above tests are OK it is now time to carry out an earth continuity test.
12. Touch the metal case of the unit with one of your test probes and at the same time touch the earth pin of the plug with the other probe. There should be a zero Ohms reading which means a full reading when using an analogue meter. If this is so proceed to the next instruction.
13. Repeat the above procedure touching the neutral pin and earth pin at the same time with your test probes and repeat this test by touching the live and earth pin with your probes. If you do not get any reading whatsoever at this stage then the above tests have gone OK and this power supply unit will be absolutely safe to use when completed.
14. If all the above procedures have gone OK it is now time to wire up the circuit board.
15. Please refer to the wiring and circuit diagrams in pictures 3 and 4 taking care that you observe the polarity of components such as the bridge rectifier and the C1 electrolytic capacitor. The flat side of the regulator must mount flush with the circuit board and a heat sink is not required because the current demand of this tuner is so low that it hardly gets warm.
16. The circuit board can now be mounted into the case as shown in picture 3 and the output side of the power supply can now be wired up.
17. Make sure you use the white 6 Amp cable for wiring the 6.3 Volt output and you must lightly twist these leads to cancel mains hum.
18. Make sure that the 0V negative side of the 25V HT output is connected to the mains earth at the solder tag bolt of T1.
19. If you are confident you have gone through all the above procedures correctly it is now time to test this unit where the greatest of care must be taken.
20. Set your meter to the DC volts range of around 50V and using crocodile clips on each probe connect the red and black probes to the 25 volt DC output observing the correct polarity.
21. Try to use a mains socket that is at least a metre clear of the unit when first switching on in case of the event of C1 exploding due to incorrect polarity. Also be very careful not to touch any mains circuitry within the unit as it will be live.
22. Switch on and stand clear for a short moment and if all is well you should get a DC reading on your meter.
23. If it is only say 10 Volts, rotate RV1 clockwise until you get the correct reading of 25 Volts DC.
24. If all the above tests seem ok, switch off the unit and set your meter to the AC volts range of about 20V
25. Repeat the above procedure but this time connect your probes to the 6V AC Output in ether polarity.
26. You should now get a reading of around 6.3 Volts off load.
27. If all these tests and procedures have gone OK, the construction of the power supply is now complete and you can now go ahead with constructing the FM Tuner.
Step 4. Preparing the Chassis
Preparing the chassis of a valve receiver and wiring the valve heaters is the most time consuming of constructing the entire receiver and if rushed and not done properly it can result in poor performance and instability. It is best to have all the components to hand that are physically mounted on the chassis before proceeding so you can familiarize yourself with them and determine the hole sizes for each component before carrying out the drilling. Before mounting any component make sure every hole required for drilling has been carried out so all the components can be mounted in one go. You must also make sure the chassis is clear of any debris such as metal filings before even attempting to install any component or short circuits may occur. Please read the following steps and guidelines before proceeding.
1.Please refer to picture 7 which is the wiring diagram. This diagram is also the best recommended layout of this tuner.
2. Start drilling all the required holes for V1 to 7 including the holes for the 2 6BA bolts of each holder. A 22.5 millimetre cutting tool is required for all the B9A nine pin holders. The V6 B7A holder requires a 16 millimetre cutting tool. Unfortunately these tools are not easily available as they once were and you may have to resort to filing out the correct size hole. Allow at least 25mm space 1 inch between each holder when mounting.
3. The front panel hole for the tuning capacitor can now be drilled along with the other holes for all the back panel sockets.
4. If you are confident all the drilling is now completed you can now go ahead with mounting all the required components
5. Chassis points marked with an E are essential earthing points which connections are usually made by a solder tag bolted to the chassis. You must make sure all these connections are clean with no grease and are physically screwed tight to the chassis or sparking may occur. Also it is advisable to have at least 3 solder tags on each bolt in which the valve holders are mounted to so neat and sound solder joints can be carried out without say 6 or more leads crammed onto 1 joint that can lead to dry solder joints, an avenue for poor performance.
Step 5. Wiring Up The Valve Heaters
1. Please refer to picture 8 which is an individual wiring diagram of the valve heater connections.
2. As you can see, this may seem an unusual method of connecting the 6.3 volt AC heaters as there leads are usually twisted to cancel mains hum and this idea came from a VHF design featured in a 1960s Practical Wireless magazine using this same exact method that led me to give it a try and as it turned out to be it seems to work very well with no traces of mains hum. The other advantage is it makes the wiring less crammed and the 0.1uf ceramic disc capacitors connected across each heater help to suppress modulation mains hum as well as there role in helping to prevent VHF oscillations from circulating through the heater line.
3. Start by following the wiring diagram very carefully and try to avoid dry solder joints on the solder tags. Double check that all solder tags are very tight and that there is no sign of any movement. 1.5 Amp tinned copper wire can be used for interconnections to SK2 but you must make sure the 6 Amp size wire is used on connections between SK3 and the Chassis ground.
4. There is 1 important point concerning the heater wiring of the audio preamplifier valve ECC82 V7. It is centre tapped at pin 9 and wired for 6.3 Volt operation rather then 12.6 Volts so you must always make sure pin 9 is at earth potential. Pins 4 and 5 are usually parallel wired but because the V7A half triode is only needed at the moment you must only connect to pin 4 or the cathode coating will become poisoned due to no electron emission. V7B can serve as a driver for an additional power amplifier using an EL84 Valve which will be featured as an add on item.
5. If you are confident everything has been wired correctly regarding this stage you may give it a test.
6. Connect the SK2 and SK3 heater terminals of the tuner chassis to the SK1 and SK2 6.3 Volt output of the power supply using lightly twisted 6 Amp wire.
7. Insert all the valves of V1 to V7 in there referring holders.
8. Switch on the power and within 1 minute all the valve heaters should start to glow red.
9. If this is the case, everything has gone well so far and you can now proceed to the initial wiring of the tuner.
9. Switch off the power and allow the valves to cool for around 15 minuets.
10. Remove all the valves and put them back into there protected cartons until they are next needed.
11. Please refer to step 6.
Step 6. Wiring Up The Main Tuner
If you have followed all the above procedures correctly you can now go ahead with wiring the main FM Tuner. If you are familiar with the testing and setting up my Transistor FM Superhet Receiver design, unfortunately you can't with this valve design test each stage as you build like you could with that design. The reason being it means reinserting the valves several times which may weaken the glass structure around the pins due to stress and the electrolytic capacitors may hold a charge in which you may encounter not so much of a shock at this low voltage but a few sparks if you accidently short these components to ground.
1. Please refer to pictures 5, 6 and 7 for the complete circuit and wiring diagram of this receiver and the Components List Of 6 Valve VHF/FM Tuner
2. There are 2 slight errors regarding duplicate numbering of the following components on the original circuit diagram of picture 5 that were corrected in the wiring diagram of picture 7 and Components List Of 6 Valve VHF/FM Tuner which I will explain. The anode load resistors of V3A and V4A was incorrectly marked as R12 and R17 in the original circuit diagram of picture 5. These have been recorrected as R12B and R17B.
3. If you are confident you understand the circuit and wiring diagram you can now go ahead with the basic wiring.
4. Try to keep all wiring neat and short as possible, particularly in the VHF circuits of V1A and V1B and do not have unnecessary long leads connecting between the tuning capacitors and RF coils or this may lead to poor performance and loss of frequency coverage at the high 108MHz portion of the FM Band. Make sure you pay attention to the polarity of the electrolytic capacitors for the same reasons I mentioned in the construction of the power supply circuit. Also try to avoid dry solder joints and if possible try to leave the heat sensitive components until last such as the small 1/4 watt resistors.
5. Tinned copper wire is best for all the interwiring of all these stages and it pays to check your wiring now and again in each stage for short circuits such as solder bridges or leads crammed close together that are a very likely cause. The points marked E as I previously mentioned are chassis earth points and also form the 0V earth return for this tuner and you must double make sure these bolts that hold the solder tags are very tight to avoid backlash or other instabilities such as sparking.
6. The RF Coils L1 and L2 are self supporting and 18 SWG tinned copper wire must be used for there construction. If this wire is hard to obtain then 1.5 Amp lighting cable with its conductors striped will do the job just as well. I have decided that the enamelled copper wire used in the Transistor FM Superhet Receiver is not suitable particularly for L2. L1 requires about 5 turns with a diameter of 9mm and a tool such as a AAA size penlight battery will surface. L2 requires the same number of turns but is tapped at 1 to 2 turns to form the cathode tap and normal tinned copper wire must be soldered to this tap to avoid stress on the valve holder pins of V1B
7. If you are now confident that you have wired everything correctly you may now refer to step 7 the initial testing and setting up of this FM tuner.
Step7. Testing And Setting Up
This is where the big moment of truth begins. Fortunately there is not the usual high voltage electric shock fears associated with this tuner, like there is with circuits that involve the usual 250 volt HT line. Valves are very electrically robust compared to solid state circuits and the only costly mistake due to connecting this tuner with wrong polarity to the 25 Volt HT line is damage to the electrolytic capacitors. The other common mistake is getting the pin 6 anode pin of each triode confused with the pin 5 heater connections which will immediately burn out the heater of the valve concerned but this is less likely if you have gone through the correct procedure of step 5.
1. Reconnect the valve heater circuit in the same procedure as previously described in step 5.
2. Using the normal black insulated tinned copper wire, connect the SK3 Black 0V terminal of the power supply to the SK5 Black 0V terminal on the back panel of the tuner
3. Using the normal Red insulated tinned copper wire, connect the SK4 25 Volt HT terminal to the SK4 25 Volt HT terminal on the back panel of the tuner.
4. As a final reminder double check all wiring and the polarity of all the electrolytic capacitors making sure the positive + corresponds as shown in the circuit and wiring diagram.
5. Insert all the 7 Valves.
6. Connect the audio output to any suitable amplifier and make sure it is turned on and the volume advanced around halfway.
7. Connect a suitable FM antenna to the SK1 coaxial input preferably an outdoor or indoor Dipole of about 1.5 metre will do for the time being.
8. Reconnect the power taking the same precaution as described in the construction of the power supply in step 3.
9. Within a minute as the valves get warm you should hear some static hiss which means there are good signs of life.
10. Try rotating the tuning control VC1 and it may be possible to receive a few FM stations.
11. If this is so try adjusting the TC1 aerial trimmer for maximum signal strength.
12. If all the above procedures have gone to plan it is now time to set and align the frequency coverage
13. If you are familiar with tuning the UK FM band, depending on your state of origin and know how the stations are located on the dial then the setting up can be done without the aid of a signal generator.
14. In the instance of the UK FM Band, try to locate BBC Radio 2 which is at the bottom end of the band around 87.5 to 90.2 MHZ.
15. If you find you are tuning Radio 2 with the VC1 vanes over halfway open then this needs correcting which I will describe.
16. TC2 in series with the VC1 tuning capacitor sets the frequency coverage. When the vanes of TC2 are fully open it lowers the capacitance of VC1 and sets the frequency coverage in the high frequency direction. The opposite is true if you have the vanes of TC2 fully closed. You are then setting the frequency coverage of VC1 in the lower frequency direction because you are effectively making the capacitance of VC1 higher.
17. To correct the case of you receiving Radio 2 with the VC1 tuning capacitor vanes half way open we need to gradually open the vanes of TC2 and keep retuning radio 2 until you can receive it with VC1 vanes open around a quarter of the way. You may find when fitting the bottom aluminium panel that this may also alter the frequency coverage slightly and may need a bit of experimentation to correct. This is due to magnetic effects.
18. If the above alignment has gone OK, try tuning a weaker local station further up the band say around 95 MHZ which is just past BBC Radio 4, the middle of the FM band.
19. Readjust TC1 the aerial trimmer for maximum signal strength. This should now correctly set the tuning range and by tuning further up the band you should be able to tune in stations such as Classic FM at equal strength with no background hiss or distortion.
20. If you are finding the V1B mixer and oscillator come to a holt when you are most likely to be tuning around the lower part of the FM Band you need to do the following procedure.
21. Increase the Cathode tap from 1 turn of the earthy side of L2 to around 2 turns. If this does not improve things much, try reducing the CX aerial coupling capacitor to a lower value of around 5PF or replace with a 2-10PF Trimmer. Also having the power supply set below 20 Volts can also be a likely cause so it may be as well check this before carrying out these procedures.
22. If everything has gone well to plan you should now have a complete working valve FM tuner of all your own work and I wish you all many happy hours of listening enjoyment. Remember nothing beats the cosy warm glow of a valve on cold winter evenings that solid state does nowhere archive
22. An optional power amplifier circuit with its own HT power supply circuit is described in pictures 9 and 10. It is capable of giving a decent loudspeaker volume of about 200 Miliwatts. Please refer to step 8 for further details if you wish to include this item.
Step 8. Adding The optional Power Amplifier
An optional power amplifier using an EL84 Output valve has been included for those who want to use the FM tuner as a portable anywhere in the home type receiver and although not HI FI quality it is capable of giving a output power of about 200mw about the same volume of the old battery valve receivers that use a DL96 pentode. Unfortunately there is not enough cathode emission when using a 25 Volt HT line to get good results regarding decent volume when using a speaker. A separate 67 Volt power supply is needed and is described in picture 10. It must also be built in the same case as the existing 25 Volt power supply or can be mounted on the receiver chassis provided it is big enough to allow room. The 67 volt HT line needs a bit more precaution then the 25 Volt HT line you are using to power the tuner and preamp section. Although the 67 Volt HT line is still nowhere as dangerous as the normal 250 Volt HT line you can get some nasty jolts if accidently touched so do take care with this stage of the construction. You also need to refer to picture 9 which is the circuit and wiring diagram combined together. The other half triode of V7B is also used for the driver section which means pin 5 of the valve base must be strapped to pin 4 to enable the heater of the V7B triode section. To save money on buying expensive output and HT transformers for this circuit the following link Maplin Electronics have the 2 transformers in question and work very well with this amplifier circuit. The output transformer is a 100 Volt line 2 watt PA transformer Code N85CC at the time of writing. The mains transformer is a 24.0.24 Volt 6VA miniature transformer Code N98CC and will power this amplifier very comfortably with no overheating effects. Also 240 Volt to 12 Volt mains transformers may also work with this circuit if the recommended output transformer is for some reason hard to obtain. There is a useful link 6BM8 Valve Audio Amplifier which has some detailed information on using line and mains transformers for valve output transformers and talking of the 6BM8 Valve audio amplifier, it is the Mullard ECL82 version and will also work very well if not better at 67 Volts HT then this EL84 version. Please refer to the Components List Of 6 Valve VHF/FM Tuner for the other components for this amplifier. This now completes this page and there will be an improvements page regarding issues such as an improved RF Stage, AGC and the possibility of getting this tuner to work with the Special FM Stereo Decoder Circuit For Pulse Counting FM Receiver . I have indeed got some good news for those wanting to have the receiver and power amplifier working at an all low voltage 25V HT Line. I have compiled a new power amplifier circuit in picture 11 based on the ECC86 triode valve and it gives a decent loudspeaker volume of 100 milliwatts audio power. Please read below for more details.
Alternative 25 Volt Optional Audio Amplifier using ECC86 Double Triode Valve
A new circuit based on the ECC86 double triode valve has been tried with great success as a low voltage power amplifier and as a result, it is now possible to obtain a decent 100 milliwatts of power using the same 25 Volt HT line as the tuner and will drive a speaker at very acceptable audio quality. More about this valve. The ECC86 was designed in the 50s as a VHF Mixer and oscillator in hybrid car radios using the space charge valve technique to eliminate the high voltage vibrator supply. It is capable of dispatching around 10 milliamps per triode using a 30V HT supply and is a framed grid valve similar to the ECC88 with its pin connections and heaters exactly the same. One word of warning. The ECC86 is not an substitute for the ECC88, particularly in the VHF Mixer of this tuner as past experiments revealed poor sensitivity compared to the original ECC88 configuration. Please refer to picture 11 which is the complete circuit and wiring diagram of this amplifier. The circuit for the driver stage uses the V7B half triode and pins 4 and 5 heater connections must be strapped together. Although the driver stage is the same circuit as used in the EL84 output stage, the HT supply to this stage must be connected to the regulated 25V HT line. The output stage has its triodes connected in parallel and is cathode biased at 0.68 volts to give a healthy anode current of around 7 milliamps resulting in low distortion. The HT supply to the output stage must route from the C1 reservoir capacitor positive connection of the power supply rather then the regulated supply for stability reasons and as with C1 having a capacitance of 1000 microfarads extra filtering is not necessary but could be a worthwhile improvement. The ECC86 at the time of writing is available from the following link Watford Valves and the output transformer is a 100 volt 2 watt line output transformer available from Maplin Electronics Code N85CC. This amplifier is more recommended then the present EL84 circuit and has a really loud volume for its low 25 volt supply which would have served well as a kitchen radio for listening to Tony Blackburn's Radio 2 Saturday morning show during breakfast in the 1960s.
Slight essential modification of this design. Please read the following notes
There has been a slight modification to this FM tuner circuit that has resulted in better performance regarding sensitivity and a much more improved frequency response curve in the IF Stages. The modification in question is the 4K7 anode load resistors, that is R8 R10 R12 R13 R17 R18 and R20. These resistor values must be increased to 15K. I will when time allows update the circuit diagram and components list.
How to enable this tuner for stereo reception
Although this tuner gives very good clean mono reception with excellent sensitivity, you may wonder will this tuner work in glorious stereo. The answer is yes providing you have a decent antenna system and are lucky enough to live in a very good reception area, close to the transmitter site as possible with no hills or tall buildings within the signal path. The answer is no if you relie on using a short piece of wire which may work very well when receiving mono reception but may result in disappointing results when listening in stereo. I would like to warn you of the following problems you may find which I myself have experienced when testing this design for stereo reception. On the very strong stations it is capable of very good results with virtually no background hiss but the only disappointment I will comment on is if you are a Radio 3 listener you may experience slight background distortion in quiet passages of music accompanied with birdies which may not be apparent when listening to mono transmissions, so if this is your only favourite station then I would think very carefully before proceeding or have a switching circuit to re enable mono if the results are disappointing. Stereo reception also relies on a signal of at least in between 70 to 80 decibels for reliable hiss free reception and you need the best antenna system you can possibly go for, that is as high as possible and clear of buildings. The best antennas to go for are the 6 element yagibeam directional type which are sadly no longer available in UK local DIY shops due to the popularity of DAB digital radio which I will mention no further. Avoid the omidirectionl type that is rounded as these antennas tend to receive signals in all directions resulting in poor results even when receiving mono reception with multipath distortion. The modification required to the tuner is very simple and picture 12 explains how to modify the filter circuit in the FM demodulator which I will describe. C26 must be reduced to 100PF to recover the the 19KHZ pilot tone and 38KHZ subcarrier. You find after doing this modification the frequency response is a bit flat when not using the stereo decoder, so it may be worth incorporating a switching facility to change this capacitor back to its original value when not receiving stereo transmissions. You must also refer to picture 13 which is a phase correction circuit which must be connected between the tuner output and the stereo decoder multiplex input. The role of this circuit is to advance the S Signal components relative to the pilot tone giving an improvement in 30db stereo separation. Without this circuit the separation will be very poor due to the nature of the pulse counting detector and low 250KHZ IF frequency which has a low bandwidth compared to the usual 10.7 MHZ IF found in commercial manufactured tuners which is the main drawback of converting this sort of tuner to stereo regarding performance. Picture 14 is the recommended stereo decoder circuit for this tuner using the MC1310P IC and you must refer to the following link Special FM Stereo Decoder Circuit For Pulse Counting FM Receiver which has full information on how to set up this circuit. Please refer to the following instructions on how to adjust the phase correction circuit.
1. VR1 is the phase shift control and advancing this preset at zero resistance has no effect.
2. VR2 is an input attenuator and also helps higher the frequency response resulting in wider bandwidth required for successful decoding of stereo broadcasts
3. Advance VR2 at full resistance
4. Advance VR1 starting from zero resistance until separation improves
5. Advance VR2 very slowly in the low resistance direction and they may be even further improvement in separation.
6. All I can say for now is if all the above steps have gone OK with the setting up of this circuit is happy stereo listening.
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