Had a similar problem with a ASL tube amp I had. I think you need to replace the bias pot.
Bias problem with ASL integrated
For the past nine months or so I've been using an ASL AQ1001DT integrated. This is a 50 watt (pentode) KT 88 design with a built in bias meter. I love the amp and it sounds mind-meltingly good with my Triangle Antal XS, although I caught the GMA bug and am now auditioning the famed Europas (their geometry reminds me of that huge furry monster in the old Warner Bros cartoons, Harrietta or Henrietta or something like that--but I digress).
Anyway, I'm having a problem with one of the tube sockets. I've tried switching tubes within the quad of Electro Harmonix I'm using now and I've also tried going back to the stock Chinese tubes. Every time I get the same effect, to some degree or other, which is that the tube in this particular socket won't hold bias. The needle creeps towards the "red zone" over and over. A little fiddling with the adjustment screw and it will drop suddenly (accompanied by a soft "pop" from the left speaker), then start climbing again after 10 seconds or so. I've been able to tease it into the right position every now and then, but it doesn't seem wise to rely on this technique indefinitely.
The first time this happened, I powered up the amp and noticed a buzz getting louder in the left speaker, accompanied by a scary orange glow from the metal cage in the middle of the tube in that slot! Freaky...I quickly shut off the amp, and with careful attention to the adjustment screw, that hasn't happened again, but it seems like a bad sign.
I am pursuing having the amp serviced by Divergent Tech, but as I am a total tube newbie, I thought I'd ask about this--maybe this is symptomatic of a common tube amp gremlin that other audiogonners would like to share their experience with.
Or maybe I've just written a small novel for no readily apparent reason :)
Anyway, I'm having a problem with one of the tube sockets. I've tried switching tubes within the quad of Electro Harmonix I'm using now and I've also tried going back to the stock Chinese tubes. Every time I get the same effect, to some degree or other, which is that the tube in this particular socket won't hold bias. The needle creeps towards the "red zone" over and over. A little fiddling with the adjustment screw and it will drop suddenly (accompanied by a soft "pop" from the left speaker), then start climbing again after 10 seconds or so. I've been able to tease it into the right position every now and then, but it doesn't seem wise to rely on this technique indefinitely.
The first time this happened, I powered up the amp and noticed a buzz getting louder in the left speaker, accompanied by a scary orange glow from the metal cage in the middle of the tube in that slot! Freaky...I quickly shut off the amp, and with careful attention to the adjustment screw, that hasn't happened again, but it seems like a bad sign.
I am pursuing having the amp serviced by Divergent Tech, but as I am a total tube newbie, I thought I'd ask about this--maybe this is symptomatic of a common tube amp gremlin that other audiogonners would like to share their experience with.
Or maybe I've just written a small novel for no readily apparent reason :)
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