UK Vintage Radio Repair and Restoration Discussion Forum - View Single Post

Neil Purling · 6th Aug 2006, 4:22 pm

The only design of TRF I have seen so far with a RF amplifier was one using EF91's as RF amp, detector and AF stage. I guess that the age of the design was when there were large numbers of these valves ex-equipment, and you could get some off your local TV engineer very easily. However the circuit uses commercial coils that are no longer available.

Maybe it's a better idea to stick with a lower gain valve on its own. I know that the EF39 works well. The EF91 did work but it was not as good in this role where regeneration was involved. I found it to be microphonic as well.
I suspect that the EF39 would be fine used on a low frequency.
Any opinion on the tuning coil?
The lowest frequency of the Long Wave band is around four times lower than the low frequency end of Medium Wave. For Medium wave I had 65 turns, tapped at 5 turns. In winding a coil four times larger I tapped it at 20 turns. The point where both coils are tapped is around 1/13th of the total number of turns.

6th Aug 2006, 4:22 pm	#48
Neil Purling Nonode Join Date: Jul 2005 Location: Hull, East Yorkshire, UK. Posts: 2,087	Re: Regarding home made TRF The only design of TRF I have seen so far with a RF amplifier was one using EF91's as RF amp, detector and AF stage. I guess that the age of the design was when there were large numbers of these valves ex-equipment, and you could get some off your local TV engineer very easily. However the circuit uses commercial coils that are no longer available. Maybe it's a better idea to stick with a lower gain valve on its own. I know that the EF39 works well. The EF91 did work but it was not as good in this role where regeneration was involved. I found it to be microphonic as well. I suspect that the EF39 would be fine used on a low frequency. Any opinion on the tuning coil? The lowest frequency of the Long Wave band is around four times lower than the low frequency end of Medium Wave. For Medium wave I had 65 turns, tapped at 5 turns. In winding a coil four times larger I tapped it at 20 turns. The point where both coils are tapped is around 1/13th of the total number of turns.