Amps will not hold bias.

@clshaw57,

I assume the power tubes are good. (Power tubes, at least 6550 power tubes, as they near their end of life bias will drift.)
I assume you have biased these amps in the past without any problems.
I assume you let them warm up at least 15 minutes, without any signal applied, before biasing them.

With both amps acting the same way they do, the only thing I can think of is the AC mains voltage is changing not holding steady. Are the 2 amps plugged directly into the wall outlet? If not try plugging them directly into the wall outlet. You might want to check the AC mains voltage at the outlet with the 2 amps powered up to make sure the Line voltage is maintaining a steady voltage.

Jim

Thanks Jim
And this is why I am so confused. It would seem that the tubes are clearly the issue but 3 pairs of 300B tubes? And of course I did not want to buy another pair in the hopes all would be well.  I was hoping that there was an obvious technical issue that would explain the problem. But? They did bias and hold steady prior to displaying this problem. This issue has occured in 2 seperate homes, I just moved. Plugged directly into wall or conditioner, with 3 different set of 300B tubes and of course every other OCD audiofool thing I can think of. And the bias problem is strange as it is actually difficult to get it to set to target 83mva but can get close then will drift drastically up or down depending on which way the bias was being adjusted.

Make sure you have speaker wires hooked up.

Make sure no signal is going in. Like, no music if pre amp is on.

You mentioned, "83 mA", but- not how much drift and/or over what period on time. "Drastically" can be quite the subjective term.