Bias trim pots no longer do anything

the thermionic watercooler

Bias trim pots no longer do anything

Postby Lafish » Fri Dec 27, 2019 2:03 pm

Posting here instead of the ST-35 forum because this one is more active. But the bias pots on my Rev. B St-35 no longer seem to do anything. Bias stays in the vicinity of 350mV, plus or minus less than 10 percent, no matter where I turn them, and with the power switched off, resistance at pin 3 likewise stays at about 450 ohms. This is not a huge problem, as the readings are close enough, and the amp still sounds fine. But it's a curious thing. I kind of doubt that all four pots have failed at the same time. The amp has been in more or less constant use since I built it in 2002 or so.
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Re: Bias trim pots no longer do anything

Postby Thermion » Mon Dec 30, 2019 7:01 am

I have a few questions for you.
1.) What revision is your ST35 PCB?
2.) Since 2002 how often have you changed your power tubes?
3.) When you say you are measuring 450 ohms is pin 3 referring to the pot or the power tube socket?

It is unlikely that your pots have gone bad. As tubes age, they get limited in the amount of current that can flow through them. If your tubes are fairly old with lots of hours on them, then it is possible that 35ma is all that they can do.

If you replace your power tubes regularly, then your problem becomes more mysterious but I am sure we can get it solved.

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