Amps will not hold bias.

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.

Are these two separate amps with their own power supplies?
If yes, what are the odds both amps would be experiencing the same problem?

Jim

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?

@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