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G6Tanuki · 5th Nov 2017, 7:51 pm

"Reducing the volume to near to nothing stops it so it has to be before the EL41."

This makes me think you've got IF signal leaking into the audio-chain, where it will be amplified [an EL41 is good for a couple of MHz] and fed back into the HT line.

Remember that the HT line is only decoupled by an electrolytic or two - which look like inductors at anything more than a few tens of KHz. So your HT line can be 'alive' with hundreds of millivolts of IF!

Try 500pF from the wiper of the volume-control pot to earth. And 100pF from the grid of the EL41 to its cathode. That should keep unwanted IF signals from building-up along the audio chain.

Though some will disagree, I like the "tone corrector" on the output-stage to be from anode-to-earth - that way it shunts any stray IF signals safely to earth. Putting it from output-stage-anode to the HT side of the output transformer feeds any stray IF signals into the HT line, just where you don't want them!

5th Nov 2017, 7:51 pm	#2
G6Tanuki Dekatron Join Date: Apr 2012 Location: Wiltshire, UK. Posts: 14,007	Re: Oh No! Not another howler. "Reducing the volume to near to nothing stops it so it has to be before the EL41." This makes me think you've got IF signal leaking into the audio-chain, where it will be amplified [an EL41 is good for a couple of MHz] and fed back into the HT line. Remember that the HT line is only decoupled by an electrolytic or two - which look like inductors at anything more than a few tens of KHz. So your HT line can be 'alive' with hundreds of millivolts of IF! Try 500pF from the wiper of the volume-control pot to earth. And 100pF from the grid of the EL41 to its cathode. That should keep unwanted IF signals from building-up along the audio chain. Though some will disagree, I like the "tone corrector" on the output-stage to be from anode-to-earth - that way it shunts any stray IF signals safely to earth. Putting it from output-stage-anode to the HT side of the output transformer feeds any stray IF signals into the HT line, just where you don't want them!