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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.
Follow Ups:
Thanks for the ideas. The schematic calls for a plate voltage of 400v with bias of -16v. I think maybe the negative voltage is throwing me off--changing the bias from -16v to -22v is actually a reduction, not an increase, in bias, causing the B+ to increase, which puts more stress on the filter caps. I guess the 457 plate voltage may be ok with the new lower bias setting (i think the max spec on the 7591A is 550v). Looks like I'm going to have to replace the filter caps. Thanks again for your help and happy new year!
with regards to bias, I like to calculate the actual current flowing through the output tubes rather than relying on the bias voltage. As the tubes age the bias voltage will have to be adjusted to maintain the correct current flow through the tubes. Is there a cathode resistor on the output tubes? If so then you can measure the voltage on the cathode and calculate the current (should be around 40 mA per tube). Also 457 volts on the plates may be a tad high? If so you might consider putting a thermistor in each leg of the primary windings of the power supply transformer to get it down to speck (if indeed it's off a bit). After following this forum for a while now it seems most people do replace the filter caps as a precautionary measure so it's probably prudent to do so. Good luck!
Dave,When you reduce standing current(output bias)the B+ will rise.The PS can caps and others will be working at their limit(leakage current)because of this and you'll get hum.If you replace the doubler caps,use sections rated at 350-450 vdc.When replacing 500vdc sections it may be OK to use new 500 vdc units but try to use 525vdc sections if possible to replace them.
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