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M-60 dc offset/ bias run amok

Posted by acres verde (A) on November 16, 2004 at 13:57:43

While listening the other nite the right channel gets ragged around the edges and distorts on transient peaks. Quick check reveals dc offset and bias are both way overboard and far beyond anything I've experienced due to 6AS7 failures in the past. After shut down, visual inspection of 6AS7s reveals no triode failures. Visual inspection of 6SN7s not possible and no tube tester is handy so a swap of all driver tubes to other M-60 is performed, the amp powered up and dc offset/bias checked and, voila, both are wildly off. In go new drivers and, after appropriate warm-up, dc offset and bias are fine tuned uneventfully. So far so good. Next day I borrow a Hickok 532 to test suspect 6SN7s. All test out beautifully, not gassy, nothing. Now I'm stumped. Anybody got an idea? Incidentally, I did take the time to test all the 6AS7s with no failures to report.

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