Home brew TRF - Page 2 - UK Vintage Radio Repair and Restoration Discussion Forum

Ed_Dinning · 4th Nov 2005, 10:36 pm

Hi Lee, some of the early TRF's used a variable cap in series with the aerial as Quantim says. This was very useful to peak up faint signals. It may also be worth adding about 1000 ohms in series with the tone caps as they get "thumping" in this position and it could save athe output transformer; alternatively use a fixed cap and a 50K ish linear pot.
Nice circuit.

Best regards, Ed

Alf · 4th Nov 2005, 11:10 pm

Another suggestion Lee is to connect the primary of the output transformer to the junction of the 1N4007 47µF and 1K resistor, just under the d in wirewound in your circuit, this was a common practice with commercial radios which will allow cooler running of the 1k wire wound resistor.

Heatercathodeshort · 5th Nov 2005, 1:22 pm

Hello,
Have a go with this 'electron coupled' circuit for a detector. It works very well and it uses a 50k pot as reaction control rather than a fiddly capacitor. It never fails to work with any R.F. pentode and the component values, H.T. potential and layout are in no way important. The coil can be homewound on any former around 1 1/2" in diameter with whatever 'thin' grade of wire you have in your junk box. About 30 ft of aerial should bring in a good few stations. Ideal simple circuit if you tend to build things and they never work! This one will. I have attached the circuit together with a shot of a '1950's schoolboy radio' I built for the local historical society. They asked me what scoolboys did in the 1950's and when I told them that some of us built radios and actually plugged them into the mains, the organizer did not believe me so I built her this 'breadboard' trf set using bits from the junk box in typical schoolboy style!
The detector circuit is followed by a 6J5 and a 6V6. Its very LOUD!
Have fun. Regards. JOHN.

dave_n_t · 5th Nov 2005, 3:17 pm

I can vouch for the efficacy of the electron-coupled detector. It's the basis of my 'PFL200 special' (which uses the 'F' pentode as this e.c. detector, and the 'L' pentode as the output pentode). I need about 8 feet of wire indoors to pick up the full range of UK stations (+ many more continentals after dark). The heater supply is 18V from one half of a 18-0-18 trannie; and the HT is the full 36 volts doubled, giving about 100V, which is quite adequate for the detector stage (but only just enough for the output pentode).

My aerial couples thru' a 22pF cap into the hot end of the tuning coil; the cathode tap is about 20% up from the earthy end.

I've done a bit of experimentation trying to replace the high value anode load resistor in the detector stage with a very high inductance - but I haven't found any inductors of high enough inductance to keep the bass content acceptable.

As octode said earlier in this thread, it's great fun playing around with these simple sets!

Hunts smoothing bomb · 7th Nov 2005, 12:38 pm

Hi all,

Thanks to you all for your pointers

I am definately going to have ago at the electron coupled circuit I'll give it ago tonight.
Cheers John

Best regards
Lee

dcriner · 9th Nov 2005, 1:14 am

For general purpose use, stocking just 1N4007 diodes makes sense.

I've gone one better. I bought a stash of 1N5408 diodes. They have the same PIV rating as a 1N4007, 1000V. But, the 1N5408 is rated at 3A rather than 1A. Physically, the 1N5400 series is about twice the size of 1N4000 series.

Here's the data sheet: http://www.rose-hulman.edu/~herniter...ets/1n5400.pdf

These silicon power diodes are cheap enough it seems to make sense to stock only the heaviest-duty that you're ever likely to use.

oldeurope · 9th Nov 2005, 10:55 am

Hi,
why did you take this rect with the center tapped mains transformer?
In a bridge rect the diodes get half of the voltage and this is much more
save.
Or use a EZ80 !
Your mechanical work looks very good.
In the schematic I found some mistakes.

Kind regards
Darius

quantum · 9th Nov 2005, 8:32 pm

With a leaky-grid detector (however the reaction is applied) it can also be useful to experiment with the grid resistor/capacitor combination to meet your needs as well. While a 1 meg resistor and 100pf cap are quite common, you can increase the resistor to up to 3.3meg and reduce the cap down to 25pf. The latter combination can increase the amplification factor and improve selectivity, but can make the circuit more unstable and difficult to control with reaction, and the signal handling capacity is reduced. However, it can be useful for short wave reception.

Neil Purling · 10th Nov 2005, 2:46 pm

How about a battery valve like a DF96? I have got a few NOS looking for suitable employment...
What changes would I need to make to component values - if any?

Heatercathodeshort · 10th Nov 2005, 8:49 pm

Hello Neil,
This one valve circuit c/o PRACTICAL WIRELESS for October 1952 shows a simple one valve circuit for a battery R.F. pentode with reaction.
The actual circuit uses an octal DF33 but the B7G DF96 will work just as well.
Headphones need to be high impedance of course unless you want to use an output transformer. The H.F. choke can be very easily homewound with some fine wire. I expect someone on the site will give you some winding details. The circuit is for L.W. reception as well but as a start you can leave out the L.W. section of the coil if you wish. It works very well and again a good start if its an early construction project. More details available if you need them. Regards. JOHN.

Neil Purling · 12th Nov 2005, 2:59 pm

Am I right by suspecting that the Practical Wireless circuit used a ready made coil by the likes of Denco?
As the circuit in the sketch used a home made coil I would ask: Is it 100 turns on a bog roll tube, Cover that with a paper sleeve and wind the 65 turnson the sleeve, tapping at 5 turns. Then the second winding may be slid up and down for best effect.

Heatercathodeshort · 13th Nov 2005, 4:41 pm

Hello Neil,
You are spot on Neil. The P.T. design uses a makers coil but not a Denco. The coil details sound about right and should work ok.
Regards, JOHN.

Neil Purling · 13th Nov 2005, 8:52 pm

To H.S.B.:Where did you get the design of the audio stages from?
I have to put something onto the electron coupled circuit.
I had looked at a circuit involving using EF91s throughout as RF amp, detector and AF. The idea of using the same valve in all stages appeals to me. The circuit I looked at employs a solid state rectifier. Would the use of a valve prevent a sudden surge in HT voltage?
Should I have a HT transformer with two heater windings if I have a thermionic rectifier?

Ed_Dinning · 14th Nov 2005, 1:32 pm

Hi Neil, There was a design of TRF in Radio Constructor in the 50's that used all EF91's. As it will not be a "lounge" set thge lower power of an EF91 will not matter, but you will need an output transformer with a high turns ratio for correct matching.
A valve rect will certainl prevent HT surges on start up as the Rectifier will only start to conduct as the receiver valves are starting to come up.
It is usual to have seperate heater windings for receiver and rectifier valves as the rect heater is not always well insulated and can break down with catrostropic results. There are valves that are designed to be used on an earthed heater line (6X4?) but they are no immune to breakdown, a seperate winding is far better.
It is also wise to fit a fuse in the centre tap lead of the HT secondary.

HTH Ed

Hunts smoothing bomb · 16th Nov 2005, 10:32 am

Hello Neil,

The audio output stage is taken from the last project "An A.C. Mains Superhet" from the book "Radio For Boys" by E.N. Bradley, Where as the TRF first stage is taken from "A One Valve Reciever" (same book) The Tone control is my own idea I just experimented with a few different value caps and a multiway wafer switch from the junk box.

Neil Purling · 20th Dec 2005, 3:42 pm

heatercathodeshort: In the sketch for the electron coupled circuit the coil, variable capacitor, 1M resistor and 100pf capacitor appeared connected, correct?

Isoplethics do a o/p transformer and they suggest a circuit using a ECL80 as a 1.5W audio amplifier. They can also make a chassis for you if you are no metal worker. I would suggest that you buy all the major components and use them to make a mock up with cardboard.

I intend using a EF91 for the valve in the electron coupled stage.
I wondered if I would need to allow for adding another EF91 as a RF amp to feed the 100 turn coil to eliminate the need for a earth and use a basic 'throw out' aerial.

Ed_Dinning · 20th Dec 2005, 5:41 pm

Hi Neil, you may find that EF80's are better and more available, as are the B9A bases, basides which you can easily sub an EF85 or 89 if you want to modify the characteristics.

HTH Ed

Neil Purling · 28th Dec 2005, 9:24 pm

This is what I am intending to use as my amplifier, (see attachment of image).
If I make the 1 meg resistor a volume control as it suggests I wire the bottom of its track to chassis & the wiper to the junction of the 220 & 150 ohms.
So it is not as powerful as Heatercathodeshorts effort with the 6V6.
I think a watt and a half is quite enough for a radio.

4th Nov 2005, 10:36 pm	#21
Ed_Dinning Dekatron Join Date: Jan 2004 Location: Newcastle upon Tyne, Tyne & Wear, UK. Posts: 8,194	Re: Home brew TRF Hi Lee, some of the early TRF's used a variable cap in series with the aerial as Quantim says. This was very useful to peak up faint signals. It may also be worth adding about 1000 ohms in series with the tone caps as they get "thumping" in this position and it could save athe output transformer; alternatively use a fixed cap and a 50K ish linear pot. Nice circuit. Best regards, Ed

4th Nov 2005, 11:10 pm	#22
Alf Hexode Join Date: May 2005 Location: Wigan, Greater Manchester, UK. Posts: 479	Re: Home brew TRF Another suggestion Lee is to connect the primary of the output transformer to the junction of the 1N4007 47µF and 1K resistor, just under the d in wirewound in your circuit, this was a common practice with commercial radios which will allow cooler running of the 1k wire wound resistor. Last edited by Alf; 4th Nov 2005 at 11:30 pm. Reason: Error in component value

5th Nov 2005, 1:22 pm	#23
Heatercathodeshort Dekatron Join Date: Jun 2005 Location: Warnham, West Sussex. 10 miles south of DORKING. Posts: 9,147	Re: Home brew TRF Hello, Have a go with this 'electron coupled' circuit for a detector. It works very well and it uses a 50k pot as reaction control rather than a fiddly capacitor. It never fails to work with any R.F. pentode and the component values, H.T. potential and layout are in no way important. The coil can be homewound on any former around 1 1/2" in diameter with whatever 'thin' grade of wire you have in your junk box. About 30 ft of aerial should bring in a good few stations. Ideal simple circuit if you tend to build things and they never work! This one will. I have attached the circuit together with a shot of a '1950's schoolboy radio' I built for the local historical society. They asked me what scoolboys did in the 1950's and when I told them that some of us built radios and actually plugged them into the mains, the organizer did not believe me so I built her this 'breadboard' trf set using bits from the junk box in typical schoolboy style! The detector circuit is followed by a 6J5 and a 6V6. Its very LOUD! Have fun. Regards. JOHN. Attached Thumbnails

5th Nov 2005, 3:17 pm	#24
dave_n_t Hexode Join Date: Sep 2003 Location: Cheltenham, Gloucestershire, UK. Posts: 298	Re: Home brew TRF I can vouch for the efficacy of the electron-coupled detector. It's the basis of my 'PFL200 special' (which uses the 'F' pentode as this e.c. detector, and the 'L' pentode as the output pentode). I need about 8 feet of wire indoors to pick up the full range of UK stations (+ many more continentals after dark). The heater supply is 18V from one half of a 18-0-18 trannie; and the HT is the full 36 volts doubled, giving about 100V, which is quite adequate for the detector stage (but only just enough for the output pentode). My aerial couples thru' a 22pF cap into the hot end of the tuning coil; the cathode tap is about 20% up from the earthy end. I've done a bit of experimentation trying to replace the high value anode load resistor in the detector stage with a very high inductance - but I haven't found any inductors of high enough inductance to keep the bass content acceptable. As octode said earlier in this thread, it's great fun playing around with these simple sets! __________________ Dave Teague

7th Nov 2005, 12:38 pm	#25
Hunts smoothing bomb Octode Join Date: Oct 2005 Location: Wimborne, Dorset, UK. Posts: 1,407	Re: Home brew TRF Hi all, Thanks to you all for your pointers I am definately going to have ago at the electron coupled circuit I'll give it ago tonight. Cheers John Best regards Lee __________________ Lee

9th Nov 2005, 1:14 am	#26
dcriner Retired Dormant Member Join Date: Aug 2005 Location: Chicago area, USA Posts: 9	Re: Home brew TRF For general purpose use, stocking just 1N4007 diodes makes sense. I've gone one better. I bought a stash of 1N5408 diodes. They have the same PIV rating as a 1N4007, 1000V. But, the 1N5408 is rated at 3A rather than 1A. Physically, the 1N5400 series is about twice the size of 1N4000 series. Here's the data sheet: http://www.rose-hulman.edu/~herniter...ets/1n5400.pdf These silicon power diodes are cheap enough it seems to make sense to stock only the heaviest-duty that you're ever likely to use.

9th Nov 2005, 10:55 am	#27
oldeurope Retired Dormant Member Join Date: Dec 2003 Location: Solingen, Germany Posts: 727	Re: Home brew TRF Hi, why did you take this rect with the center tapped mains transformer? In a bridge rect the diodes get half of the voltage and this is much more save. Or use a EZ80 ! Your mechanical work looks very good. In the schematic I found some mistakes. Kind regards Darius

9th Nov 2005, 8:32 pm	#28
quantum Retired Dormant Member Join Date: Mar 2003 Location: Birmingham, West Midlands, UK. Posts: 785	Re: Home brew TRF With a leaky-grid detector (however the reaction is applied) it can also be useful to experiment with the grid resistor/capacitor combination to meet your needs as well. While a 1 meg resistor and 100pf cap are quite common, you can increase the resistor to up to 3.3meg and reduce the cap down to 25pf. The latter combination can increase the amplification factor and improve selectivity, but can make the circuit more unstable and difficult to control with reaction, and the signal handling capacity is reduced. However, it can be useful for short wave reception.

10th Nov 2005, 2:46 pm	#29
Neil Purling Nonode Join Date: Jul 2005 Location: Hull, East Yorkshire, UK. Posts: 2,085	Re: Home brew TRF How about a battery valve like a DF96? I have got a few NOS looking for suitable employment... What changes would I need to make to component values - if any?

10th Nov 2005, 8:49 pm	#30
Heatercathodeshort Dekatron Join Date: Jun 2005 Location: Warnham, West Sussex. 10 miles south of DORKING. Posts: 9,147	Re: Home brew TRF Hello Neil, This one valve circuit c/o PRACTICAL WIRELESS for October 1952 shows a simple one valve circuit for a battery R.F. pentode with reaction. The actual circuit uses an octal DF33 but the B7G DF96 will work just as well. Headphones need to be high impedance of course unless you want to use an output transformer. The H.F. choke can be very easily homewound with some fine wire. I expect someone on the site will give you some winding details. The circuit is for L.W. reception as well but as a start you can leave out the L.W. section of the coil if you wish. It works very well and again a good start if its an early construction project. More details available if you need them. Regards. JOHN. Attached Thumbnails

12th Nov 2005, 2:59 pm	#31
Neil Purling Nonode Join Date: Jul 2005 Location: Hull, East Yorkshire, UK. Posts: 2,085	Re: Home brew TRF Am I right by suspecting that the Practical Wireless circuit used a ready made coil by the likes of Denco? As the circuit in the sketch used a home made coil I would ask: Is it 100 turns on a bog roll tube, Cover that with a paper sleeve and wind the 65 turnson the sleeve, tapping at 5 turns. Then the second winding may be slid up and down for best effect.

13th Nov 2005, 4:41 pm	#32
Heatercathodeshort Dekatron Join Date: Jun 2005 Location: Warnham, West Sussex. 10 miles south of DORKING. Posts: 9,147	Re: Home brew TRF Hello Neil, You are spot on Neil. The P.T. design uses a makers coil but not a Denco. The coil details sound about right and should work ok. Regards, JOHN.

13th Nov 2005, 8:52 pm	#33
Neil Purling Nonode Join Date: Jul 2005 Location: Hull, East Yorkshire, UK. Posts: 2,085	Re: Home brew TRF To H.S.B.:Where did you get the design of the audio stages from? I have to put something onto the electron coupled circuit. I had looked at a circuit involving using EF91s throughout as RF amp, detector and AF. The idea of using the same valve in all stages appeals to me. The circuit I looked at employs a solid state rectifier. Would the use of a valve prevent a sudden surge in HT voltage? Should I have a HT transformer with two heater windings if I have a thermionic rectifier?

14th Nov 2005, 1:32 pm	#34
Ed_Dinning Dekatron Join Date: Jan 2004 Location: Newcastle upon Tyne, Tyne & Wear, UK. Posts: 8,194	Re: Home brew TRF Hi Neil, There was a design of TRF in Radio Constructor in the 50's that used all EF91's. As it will not be a "lounge" set thge lower power of an EF91 will not matter, but you will need an output transformer with a high turns ratio for correct matching. A valve rect will certainl prevent HT surges on start up as the Rectifier will only start to conduct as the receiver valves are starting to come up. It is usual to have seperate heater windings for receiver and rectifier valves as the rect heater is not always well insulated and can break down with catrostropic results. There are valves that are designed to be used on an earthed heater line (6X4?) but they are no immune to breakdown, a seperate winding is far better. It is also wise to fit a fuse in the centre tap lead of the HT secondary. HTH Ed

16th Nov 2005, 10:32 am	#35
Hunts smoothing bomb Octode Join Date: Oct 2005 Location: Wimborne, Dorset, UK. Posts: 1,407	Re: Home brew TRF Hello Neil, The audio output stage is taken from the last project "An A.C. Mains Superhet" from the book "Radio For Boys" by E.N. Bradley, Where as the TRF first stage is taken from "A One Valve Reciever" (same book) The Tone control is my own idea I just experimented with a few different value caps and a multiway wafer switch from the junk box. __________________ Lee

20th Dec 2005, 3:42 pm	#36
Neil Purling Nonode Join Date: Jul 2005 Location: Hull, East Yorkshire, UK. Posts: 2,085	Re: Home brew TRF heatercathodeshort: In the sketch for the electron coupled circuit the coil, variable capacitor, 1M resistor and 100pf capacitor appeared connected, correct? Isoplethics do a o/p transformer and they suggest a circuit using a ECL80 as a 1.5W audio amplifier. They can also make a chassis for you if you are no metal worker. I would suggest that you buy all the major components and use them to make a mock up with cardboard. I intend using a EF91 for the valve in the electron coupled stage. I wondered if I would need to allow for adding another EF91 as a RF amp to feed the 100 turn coil to eliminate the need for a earth and use a basic 'throw out' aerial.

20th Dec 2005, 5:41 pm	#37
Ed_Dinning Dekatron Join Date: Jan 2004 Location: Newcastle upon Tyne, Tyne & Wear, UK. Posts: 8,194	Re: Home brew TRF Hi Neil, you may find that EF80's are better and more available, as are the B9A bases, basides which you can easily sub an EF85 or 89 if you want to modify the characteristics. HTH Ed

28th Dec 2005, 9:24 pm	#38
Neil Purling Nonode Join Date: Jul 2005 Location: Hull, East Yorkshire, UK. Posts: 2,085	Re: Home brew TRF This is what I am intending to use as my amplifier, (see attachment of image). If I make the 1 meg resistor a volume control as it suggests I wire the bottom of its track to chassis & the wiper to the junction of the 220 & 150 ohms. So it is not as powerful as Heatercathodeshorts effort with the 6V6. I think a watt and a half is quite enough for a radio.