Fisher X-200 Bias Question

Vintage Asylum

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Greetings Inmates,
The bias supply on my Fisher X-200 integrated (7591 outputs) seems to be acting up. I've restored the amp following the Fisher Doc's recommendations as well as other info found on these pages, I've replaced all mylar caps, the bias supply electrolytic cap, diodes and a few drifted resistors. I also made the suggested bias supply mod, installing a lower value resistor to allow the bias to be set around -22v. I am currently using a quad of reasonably well matched GE branded 7591As.
When adjusting the bias the other day (using the meter on the chassis), the amp began to hum as I set the bias to the desired point. Thinking the meter could be faulty, I checked bias at pin 6 of the output tubes and I could not set the bias above -20.5v without the hum coming back. The hum begins around -20.5v and builds in intensity up through -23v (the top end of the range on my bias pots). The hum is in both channels and is independent of the volume control. Currently, I have the outputs balanced and biased at -20.5v, and the voltage at pin 3 is 457v for all tubes. While my bias supply will let me set the bias at the desired level of -22v, the hum is too loud at this point.
My first thought is that one of the can capacitors is failing. I don't have much experience with adjustable bias amps. My other amps have no bias adjustment and when I have had a filter cap fail, the telltale hum appears and does not go away until the cap is replaced. Here, I can get rid of the hum by reducing the bias to -20.5v. In my limited understanding, I would expect that lowering the bias to -20.5v would put more stress on the circuit thereby increasing (not decreasing) the hum from a failing filter cap.
I've combed the archives, but can find no discussions regarding this type of "attenuatable" hum in an adjustable bias supply.
As always, any thoughts are gratefully appreciated.
Thanks and Happy New Year!
Dave

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Topic - Fisher X-200 Bias Question - DHJ 08:37:13 12/30/03 (3)

Re: Fisher X-200 Bias Question - DHJ 15:06:07 12/31/03 (0)

Re: Fisher X-200 Bias Question - microbe 10:20:21 12/30/03 (0)

Re: Fisher X-200 Bias Question - scrooie looie 09:48:14 12/30/03 (0)

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