My own working version of this featured radio.	Basic circuit diagram of 2 valve TRF Receiver.	Circuit Diagram of HT Power Supply	Recommended underside chassis layout and wiring.	Front, top and rear view of chassis
Simple band switching circuit, For the entire medium and short wave bands. Excluding long wave.	Physical layout of tuning coil for each band.	RF Amplifier for this design. This stage is worth including, As it gives improvement in gain and reduces radition and aerial detuning effects	New improved HT power supply circuit for this design, With choke input filter and essential if you plan upgrading to my new 7 valve superhet receiver.

Please left click on selected image, to enlarge and print.

Construction Guide

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

R1. 150R.

R2. 4K7R

R3. 47K

R4. 1K

RV1 1M Lin, Power potentiometer 2 watt minimum.

Capacitors. Ceramic Disc or Silver Mica, 350 Volts minimum

C1A. 1000PF

C1B. 0.01uf

C2. C3. C4. 1000PF

C5. 10PF.

Inductors

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.

Smoothing Choke

10 Henries rated at 100 Milliamps. Available from the above links referring to the mains transformer  

Bridge Rectifier

1 Amp 400V minimum rating or 4 X IN4009.

Capacitors

2 X 100uf rated 350 or 450V Electrolytic capacitor

Links to my other pages within this site

Home Page

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

Important Electronic Symbols Used In Valve Circuits

Important Electrical Safety Issues Related To Valve Equipment

How To Get Components

mailto:philip@philsvalveradiosite.co.uk