Amps will not hold bias.

Thanks for asking.
Bias will drift up or down dramatically, for instance if adjusting up to desired reading they will initially hold and then continue to drift upwards subsequently if adjusting down they will initially hold then drift downward.

If you are for sure that your tubes are not bad and the socket which holds these tubes are check and inpected then your answer would be:


There could be two things.

1) Your bias pot are going bad. Clean and do measurement on both channel.  See if you get a reading.  Turning clock wise or counter clock wise will either increase or decrease.  If both has the same reading ( you are good).

Next step. 

2) one of the resistors to ground could be open or bad.

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.

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.