The well-behaved valve TRF

[Neil Purling]

I have made several over the years, with different valve combinations.
It doesn't make any difference, but they are all inclined to squeal when I want to wind the volume right up.
I assume that there's still RF content in the signal that passes through the two audio stages & somehow that is getting back to the detector.
Old TRF designs of the 1920's had the detector transformer-coupled to the audio stage/s.
So, would purchase of a transformer instead of coupling the detector to the first audio stage with a 0.01uf capacitor effect a cure?
What ratio should I buy? I see 3:1 & 5:1 ratios.
I would never buy one unless it'd been resistance tested across each winding.

[GMB]

If it is RF instability then you should see a big difference by putting a small value capacitor across the audio.

But it may well just be AF instability due to layout issues. I used to have big problems with this when I was a child, mainly because I couldn't afford to make a chassis out of metal.

Transformer coupling does make it easier to avoid such problems.

[PJL]

Well behaved TRF's are normally have good screening between the audio and RF stages.

[Ed_Dinning]

Hi Neil, quite often an RF choke was also included in the anode circuit in series with the AF transformer.

Ed

[G6Tanuki]

Another useful trick is to put a resistor of about 10K in series with the feed to the grid of any audio-stages. Wire it *right next to* the grid terminal on the valve-base with minimal lead-length.

The resistor, coupled with the few picofarads of grid-to-cathode capacitance, acts as a low-pass filter and removes the RF without unduly attenuating the audio component.

[Neil Purling]

I wondered whether there was RF getting along the HT line or not.
I added another HT transformer to the PSU: One for the detector, the other for the audio stages.
A dramatic difference, but the PSU is 50% of the chassis space.
You really do need screened cable anywhere that is carrying an audio signal:
To & from the volume 'pot.
Between the anode of the pre-amp and grid #1 of the 6V6.
I can now wind the volume right up to '10' without a single squeak, no need for a grid-stopper resistor on the 6V6.

I have got a HF choke to add in the feed to the anode of the detector.
Maybe I can strip out the extra transformer even without using the choke?

[Skywave]

Originally Posted by Neil Purling

I wondered whether there was RF getting along the HT line or not.
I added another HT transformer to the PSU: One for the detector, the other for the audio stages: a dramatic difference.

Which suggests that the original design with one common PSU had inadequate decoupling between the detector and audio stages. That unwanted coupling could have been caused by several things: inadequate HT decoupling; inadequate screening; common earth currents (poor earthing arrangements), amongst other things.

Al.