Components List And Chassis Layout For 2 Valve TRF Receiver
Components List For Basic Receiver
0.5 Watt Carbon or metal film except where stated.
R1. 2M. R9. 100K
R2. 2K2 R10. 680K
R3. 10K R11. 1K
R4. 220K R12. 150R 2 Watt wire wound.
R5. 270K R13. 6K8. 1 Watt
R7. 3K3 RV1 1M Power potentiometer Linear or wire wound, about 1 Watt minimum for regeneration.
R8. 100K RV2 1M Log pot. For volume control.
VC1 365PF Single gang for tuning. Jackson Brothers.
VC2 15 PF Trimmer for band spread and fine tuning. Jackson Brothers.
Capacitors. Silver mica or disk ceramic 500V.
C1. C5A. 100PF.
C2. C3. C4. C7. 0.01uf.
Capacitors. Electrolytic. Minimum working voltage 250-450V except where stated.
C8. C11. 47uf 25V
C9. 47uf 450V
C12. 1uf 450V
Valves and holders
2 B9A Chassis mounting type
5K Single ended type to match 8 ohm speaker. NB Available by clicking on the following links SOWTER AUDIO TRANSFORMERS HOME PAGE or a cheaper alternative, highly recommended for a low powered valve radio like this one is Multi-tapped Output Matching 3.5W AF Transformers available at RS components.
Reasonable size 4 Watts minimum 8 ohm. A good choice is a 15 Watt elliptical speaker I got from Maplin about 13 years ago, gives excellent vintage valve sound quality in a small compact space.
L1 Tuning Coil. Wire and Former
Medium Wave. 600 Kc to 2 MHZ. 36 SWG Enamelled copper wire.
SW1. 2.5 to 7MHZ 26 SWG Enamelled copper wire.
SW2. 5 to 14MHZ 26 SWG Enamelled copper wire.
SW3. 11 to 32MHZ 20 SWG Enamelled copper wire.
Former. 15mm diameter, plastic pipe available from most DIY stores such as Home Base or Focus.
Wave Change Switch
3 Pole 4 Way rotary switch.
Components List For Power Supply Using Capacitor Input Filter
R1. 150R 2Watt Carbon or metal Film. NB. If you are going to use a higher voltage HT Line, this value will need to be increased, which may call for experimentation due to regeneration stability.
R2. 100R 5Watt Wire wound.
C1. C2. C3. 47uf or better 100uf 450V Electrolytic.
D1. 2. 3. 4. IN4009 or similar Bridge Rectifier 1 Amp 400V minimum rating.
3V.0.3V 1.5 Amp minimum for 6.3V Valve heaters
24.0.24V 0.125MA Minimum, with secondary windings wired in series to give 48V
Mains Transformer for high voltage version. Same as already stated but Secondary voltage is 100V, to give a RMS of about 150V.
Switch. Double Pole 5Amp minimum toggle switch rated at 240V AC.
Fuses. 100MA Slow Blow 240V.
Cable 3core, 3Amp Mains Cable.
Hardware And Other Optional Items
Chassis. Aluminium box, available from Maplin Electronics. Code KR55K or bigger box, Code KR57M which is more recommended, should you wish to extend the receiver with a RF stage or convert it to a Superhet, at a later stage.
Bolts, Screws and washers. All available from Maplin or RS Components.
Knobs. Available from Maplin or RS Components. Style and choice is up to you.
Fuse Holder, NB Better option is Euro Style Mains inlet chassis plug and fuse holder, with insulating boot available from Maplin Electronics. Note. You will also need a mains lead, with a Euro style socket, fused at 3 amps.
Terminal Posts. Red and Black for speaker. Yellow and Green for Aerial and Earth.
Connecting Wire. Twisted bell wire 1.5 Amp minimum, for valve heaters.
Red and Black, High voltage rated tinned red and black connecting wire for HT connections, about 1 Amps.
Tag board, For interconnections on chassis. Rubber Grommets for mains cable and other interconnection cables through chassis.
Diagrams And Construction
Please left click on selected image, to enlarge and print.
1. It is best to prepare the chassis first by drilling the required holes and mounting all physical components first, such as the transformers, tuning capacitors and valve holders etc. The picture in the order of 3, from the left of this page is the suggested underside chassis layout and wiring diagram. Please note that the points marked E, as shown in the diagram are solder tags where all earth point returns are made, making the wiring easy.
2. The polarity of all components marked with a + sign, such as the rectifier diodes and electrolytic capacitors are very important and must be observed very carefully or permanent damage may result.
3. On completion of this design it is very important that some sort of cabinet or case is considered, as it is dangerous to have live mains connections exposed. Valves also run very hot and some form of ventilation must also be considered, such as small ventilation holes or grills in the top and at the back of the case.
Winding the tuning coils
The tuning coils are homemade using 15mm plastic tube, obtained from most DIY stores and enamelled coated copper wire is used, requiring a certain wire gauge depending on the band covered. You may also have to experiment with the cathode tap at points B and A by adding a few more turns then stated, if there is total lack of feedback when advancing the regeneration control clockwise. Also the reverse, If there is also to much feedback making the regeneration uncontrollable.
Coil winding details for each band covered
SW1 2.5 - 7MHz. Former length 30mm.
Top winding C to D. 6 turns.
Main winding B to C. 28 turns.
Bottom winding A to B. 9 turns.
Total 43 turns 26 SWG.
SW2 5 - 14MHZ. Former length 30mm.
Top winding C to D. 3 turns.
Main winding B to C. 8 turns.
Bottom winding A to B. 9 turns.
Total 13 turns 26 SWG.
SW3 11 - 32MHZ. Former length 35mm.
Top winding C to D. 1 turn.
Main winding B to C. 4 turns.
Bottom winding A to B. 1.2 turns.
Total 6.2 turns 20 SWG.
Medium Waveband. Former length 42mm.
Top winding C to D. Not used.
Main winding A to C. 140 turns.
Bottom winding. 20 turns.
Total 160 turns 36 SWG.
Choice of band and switching
The choice of band or weather you want to add bandswitching facilities or not is entirely up to you, but as a starter to get it up and running, I would build it as a single band receiver to start with, using SW2 as this has the most worldwide coverage of stations 24 hours. The Medium Wave Band is mainly for local and international stations, but now with almost every BBC and ILR stations only transmitting on the FM frequencies, the choice is limited to late evenings where you might find the occasional music or pop station. The fourth picture on the right is a suggested bandswitching circuit and I must state that this is where the stability of the receiver regarding backlash and other problems such as hand capacity affects can become a problem. It is particularly important that SW1 and 2 have the shortest wiring to the wavechange switch, as these carry high frequency currents and may result in loss of high frequency coverage. Also avoid coupling effects by not having the coils mounted to close together.
Testing the receiver
This is where moment of truth starts. Before proceeding please make sure you fully understand the full safety precautions I have mentioned by clicking on the following link Important Electrical Safety Issues Related To Valve Equipment. Providing the mains fuse is the stated rating of 100MA and smoothing capacitors are all connected in the correct polarity, the damage will be limited whatever mistake is made.
1. Fully Check all wiring for any possible mistakes such as solder bridges, dry joints and fully make sure all mains wiring to switch and primary side of mains transformer is correct.
2. Inset the two valves in the correct bases
3. Connect a speaker and about 1 metre of insulated wire to the aerial terminal.
4. As this is the first test, Stand about 2 metres away with the chassis pointing towards the ceiling in case anything serious occurs.
5. Switch the power on and if all is well, the valves will show a red glow and within a minute, as the valves get warm they should be signs of life.
6. If all has gone well you should by turning the volume control, RV2 clockwise be able to hear some kind of activity.
7. The first thing you are likely to hear is a soft hiss in the speaker, which means the audio stage is working
8. Turn the regeneration control, RV1 clockwise until you hear howling, which means V1a is oscillating.
9. Advance RV1 anticlockwise, very slightly until the howling ceases.
10. By rocking VC1, The tuning control in both directions you should be able to tune in a few stations
11. VC2 is a bandspread control and due to its slow motion effect you will find tuning the crowded shortwave bands much more easy particularly during mid evenings. It also acts more of a fine tuning control when using the medium waveband.
Optional RF Amplifier For This Design
An optional RF Stage has now been included for this design and the circuit diagram is featured in the 8th picture. As this circuit is very simple, it just shows the actual diagram. I have assumed that providing you have successfully built a working version of this radio, You should have no difficulties with this circuit. This circuit does not only give a worthwhile improvement in gain, Its purpose is to stop radiation from the regeneration detector, That could lead to interference on nearby sets tuned to the same frequency and reduces annoying detuning effects, When the aerial wire is moved. The brief circuit description is as follows. The 4.7mh RF choke functions as a wideband tuned circuit and reduces the need of winding an extra RF Coil and tuned circuit, That could lead to instability and stray coupling effects. The valve used is a EF91 Pentode, Introduced in the 1950s as one of the early 7 pin base miniture valves and has been used in many RF and AF applications of that period, Including FM Receivers and the Mullard 2 Valve Stereo Pre Amp. The variable resistor RV1, functions as a RF Gain control, preventing overload of the regeneration detector when using a long wire Aerial.
Components List For RF Stage
Resistors. 0.5 Watt Carbon or Metal Film
RV1 1M Lin, Power potentiometer 2 watt minimum.
Capacitors. Ceramic Disc or Silver Mica, 350 Volts minimum
C2. C3. C4. 1000PF
RFC 1. 4.7mh.
Valves and Holders
EF 91 Miniture RF Pentode.
7BA 7pin ceramic valve base
Improved Choke Regulated Power Supply For This Design
The 9th picture features a circuit for a Choke regulated power supply for this design and should give a better improvement regarding mains hum and smoother control of the regeneration, Resulting in better stability of this receiver. This circuit had been recently suggested because some of you may want to upgrade your existing TRF Receiver design to my newly featured Severn Valve HF Superhet Receiver Designed For Advanced Constructors. If you intend keeping your existing TRF Receiver the mains transformer can be retained. Please refer to the components list below.
Components List For Regulated Power Supply
New Mains Transformer
Primary 230V for UK AC Mains
Secondary 1. 1 X 6.3V CT for EL84 rated at 1 amps minimum
Secondary 2. 1 X 6.3V CT for the following 6 valves in the superhet receiver stage rated at 2 amps minimum
Secondary 3. 1 X 70V at 60MA if you intend using transistor IF Coils.
1 X 100V at 100MA only recommended when using valve type IF Coils
Total VA Rating of entire transformer is 23.1 VA when using 70V.
Total VA Rating of entire transformer is 28.9 VA when using 100V.
NB. Please ensure you state the above ratings correctly when ordering. Please refer to the following 2 recommended links to order this transformer SOWTER AUDIO TRANSFORMERS HOME PAGE or VVT Mains Transformers for Electronic Equipment You can omit this component if not upgrading to my new 7 valve superhet receiver.
10 Henries rated at 100 Milliamps. Available from the above links referring to the mains transformer
1 Amp 400V minimum rating or 4 X IN4009.
2 X 100uf rated 350 or 450V Electrolytic capacitor
Links to my other pages within this site
Simple 2 Valve TRF Tuned Radio Frequency Receiver
Introducing Superhet Receivers' Featuring Add On Converter For The Two Valve TRF Receiver
Severn Valve HF Superhet Receiver Designed For Advanced Constructors
Transistor FM Superhet Receiver
Double Conversion Pulse Counting FM Superhet Receiver With 10.7 MHZ First IF Stage
Special FM Stereo Decoder Circuit For Pulse Counting FM Receiver
Single Conversion 6 Transistor 10.7 MHZ Pulse Counting Receiver, Designed For Stereo FM Reception
6 Valve VHF/FM Pulse Counting FM Tuner Using Safe 25Volt DC HT Line
Solid State AM/FM Pulse Counting Receiver Designed For HF Short Wave Band Reception As Well As Superb Clear VHF/FM Reception
Valve Version Of The 10.7 MHZ Double Conversion VHF/FM Pulse Counting Tuner
3 Valve 3 Watt Stereo Amplifier
Important Electronic Symbols Used In Valve Circuits
Important Electrical Safety Issues Related To Valve Equipment
How To Get Components
Site Map Of All My Webpages And Favourite Valve Radio Related Links