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In Reply to: RE: Do your voltages somewhat match the schematic? (nt) posted by vinnie2 on June 21, 2017 at 13:42:38
Measure in sections: Get the power supply stable, then install and measure the the front-end/driver section, then plug in and measure the output tubes. It will help you nail down the problem.
Did like you suggested. PSU is ok, nice and steady by itself. Then I installed just the driver 12at7 tube and it's voltages are pretty much in the ballpark. I then plugged in the 6bq5's. I was getting around the 13 volts at the cathode resistor with the 400 ohm resistor in place that the schem calls for, for about 33 mA for cathode current if I am doing the calcs right. The tube manual says it should be about 65 mA, but would that be for a shared cathode resistor too? Still distorting a lot. The other voltages are in the ballpark too. I am at a loss as to what to try next. Maybe I should test some 6bq5's and !2at7's and make sure I have a good set of tubes? Maybe hook a test signal to it and follow it through on a scope and see if I can see where it starts distorting?
I think the power supply is ok because it is one from another amp, but I will check it. I will try isolating the other sections in the morning. Thanks for the tip.
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