Over the past 2 years I've had my Triode ST 70, I've very much enjoyed its (mostly) trouble-free operation. Ever since I moved a couple of months ago, I'd get the occasional bias flare up when I would leave the amp on for many hours at a time (I finally found a local radio station that plays some decent jazz and like to have it on more or less all day). Up until today, I'd notice that the bias voltage would drift slightly depending on how long the unit had been on. So if I checked bias within a minute or two of firing it up, I'd get a somewhat high reading relative to what it should be (1.10-1.18v), so thinking something was up, I'd back off the bias slightly and get it back down to spec (1.00v). After a couple hours of playing, everything would naturally be warm, and I'd get a slightly low bias (.89-.92v). If I readjusted to meet spec, I'd get a low voltage when it was warm again. I'm not sure if that is normal for component values to drift that much, or if it is even a problem.
That brings me to a few minutes ago. I had the unit on for a couple of hours, all was going just fine, then I glanced over and noted that 1 of the tubes was having some serious red plating (it got hot enough to melt the paint off the tube). I immediately killed power and dialed back both bias trim pots to fully CCW, waited a minute or two, and fired it back up. I now read .75v across both bias test points.
Right now I'm not quite sure what to think, though I reckon I've got two separate problems (one being the heat related fluctuation of the bias, and the other being bias flare-ups). Does anyone have any advice on what to look for going forward? And is the tube that went red plate garbage now? Thanks guys!