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Old 8th March 2012, 04:42 PM   #11
kevinkr is offline kevinkr  United States
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The driver stage was a bit different, used a 5751 in the first stage and a 12BH7 in the second. No balance pot in the front end. The existing design implemented with the 6CG7 should be fine.

Differences between the 12AU7 and the 6CG7 may account for some of the problems you are having, they are just a little different in terms of transconductance for a given bias and plate voltage. Changing those two resistors should get you where you need to be I think. Set the balance pot so that the plate voltage on the second stage plates matches well for best overall symmetry. (Within a couple of volts is generally OK)
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Old 8th March 2012, 05:12 PM   #12
limono is offline limono  United States
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Ok , I changed them and all is fine , thanks a bunch!!!! The amp was blowing output tubes and melting PCB board (where screen cathode resistors were installed) when I've got it. Part of it was faulty tubes but I think octal sockets are culprit and needs to be cleaned frequently.
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