Amps will not hold bias.

Almarg, where are you? Help needed here.

Hi Zipost  
1) Can I be sure that tubes are good I don't have a tester but seems unlikely that 3 pair would all display the same issue. But can anyone be sure of anything in this hobby LOL.
2) bias pots seem to adjust and track the same from amp to amp although one seems to run out of adjustment before the other. But might clean pots to see if that helps.
3) May pop the bottom off and look to see what I can see re: resistor but getting over my head at that point.

Hi Jim
They are mono amps and yes I agree with your assessment, and that is the big head scratcher. What are the odds that all 3 pair of tubes would have the same issue?

Hi Roxy54 
Ventura CA, You know someone that could help?

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.