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Playing through the amp and noticed it running hot. Checked the bias and it had shot up to 189mv. Opened it up and checked the solder joints, B+, screen resistors, bias resistor,diode, cap, heaters, all check good. Neg voltage is -68v, high?. Tried another set of 6L6's and get same high bias. 1ohm resistors good. Preamp tubes all test good. Wondering where I go next on the check list.
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What exactly does the bias voltage at the grids read when it goes haywire?
-65Vdc is considered low bias ie: low current draw.
Any 'open' in the bias circuit will send the tubes into high current draw.
Open ground reference, open adjust pot, to name a few.
You could measure the Vac ripple on the bias voltage.
That will prove out the cap.
Watch out for correct polarity of the bias cap. I have seen them installed backwards. -
I just went ahead and changed out everything in the bias section since I had the parts. So far so good. Something must have gone open when it warmed up.Comment
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