Symptom:   Amp warmed up fine in Standby.  When switched to “ON” position the amp quickly developed a low rumble that escalated immediately to an almost full amplified rumble.  I immediately switched back to Standby.

I tried letting the amp cool down and then turned it to the “On” position again.  As the amp warmed up I tapped the tubes with a wooden chopstick and all the tubes responded about the same except for one of the power tubes…which rang and the sound grew louder as the tube warmed up.   As soon as it warmed the amp immediately went into the amplified rumble…regardless of whether the volume was turned up or down.

I let the amp cool down and then removed the suspect tube.   I warmed it up again and the amp worked well sans the faulty power tube.   I was surprised…because all that I read about microphonic tubes, none of the examples sounded as severe as what I experienced.   I ordered a matched set of power tubes and I’ll see if this solves the problem.

Update:   Problem still there but the new tubes sounded clean

OK….finally did trace that low rumble feedback.   It turned out to be a triple capacitor that had also gone bad.   I was hoping not to have to replace that one which had looked like it had been replaced once.  It was 22/20/20 uf rated at 450 Volts and I really wasn’t able to find a triple electrolytic capacitor to repalce it with.     See the post labled Triple Capacitor for some photos.    Problem solved:  The amp is working!