[Diabolical Artificer]

Ere we go again 2, 3, 4... After making a few changes and improvements like changing the front end and winding another wndg on the big toroid tfmr to replace the little 45v 50mA tfmr which was getting hot and buzzing and also kicking out lots of nasty magnetic fields, the amps are essentially finished, or so I thought.

Yesterday after I debugged the clipping indicator I did a quick test with music to see if the clipping indicator worked ok with that too, rather than with a 1khz sinewave. So I got out a long lead that runs from my amp (amp1) in the living room to my workshop bench, this plugs into the headphone socket of amp1, then into the RCA of the amp(amp 2, valve amp/DUT). Switched on the amp and got oscillation out of the amp (amp1) speakers, What the flegnog! Oscillation sounds about 400hz, puzzler.

The lead in question is about 10ft and is a 1/4" stereo jack split into two RCA's, I put an AC meter on the unused RCA, in fault condition I'm reading about 3v RMS AC, it should be way lower than this, about 500mV RMS, you can see the voltage rise as the valve amp warms up. One other thing that maybe indictive, I realised I'd left off the mains earth to the amp (amp2) chassis, after reconnecting this the amp still oscillates, but there is no sound out of the living room speakers. I can only test very briefly as I don't want to damage anything, but the OP of amp 2 has what looks like a square wave of about 1v P-P. Lastly all is well with an MP3 player as source. So there must be some FB or capacitance issue, amp 2 is into a dummy load, so this isn't howl around.

Any ideas welcome. Andy.