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It came to me with 2 burned out 6v6s. Put in new tubes and the bias drifts bad....Bias adj pot does not help or settles the bias. Checking the grd. Head scratch 1.... Anybody bias one of these? Schematic at thetubestore.com
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You are checking bias at test points TP40 & TP41?
The screen resistors R142 & R143 checked good?Originally posted by Enzo
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Bad circuit? Bad tubes? It's one or the other. If it's the circuit it's most likely a bad capacitor or a dirty/intermittent connection. If it's bad tubes it's the new tubes you've installed. Less likely but not out of the question."Take two placebos, works twice as well." Enzo
"Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas
"If you're not interested in opinions and the experience of others, why even start a thread?
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i would clean very well tube sockets with ipo alcohol,both on top and on the pcb side,then check out the bias circuit for any instability,for good measure i would replace the pot,that is very critical,better use a good quality cermet pot.Originally posted by 1ampman View PostComment
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Have you looked at the output on a 'scope? It might be oscillating.
Is the bias voltage varying without the 6V6's fitted?Experience is something you get, just after you really needed it.Comment
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C85 is suspect when the bias voltage is unstable if the valves, bias pot and other components perform as they should.Support for Fender, Laney, Marshall, Mesa, VOX and many more. https://jonsnell.co.uk
If you can't fix it, I probably can.Comment
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Thx for the replies. In the mean time I burnt out the 7915 regulator- will have a replacement this week.. The test point is speced for 23 mvdc. in the cathode circuit. Also using my usual tube socket bisas/current probe...The tubes do not show current draw. or anything like a normal tube amp. I'll check the screen resistors.Comment
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