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RE: Runaway 7581A

Posted by Thermionic27609 (A) on July 3, 2009 at 05:28:40

In Reply to: RE: Runaway 7581A posted by FenderLover on July 2, 2009 at 23:10:15:

"Do all four 7581A start going ballistic, together? Or just one tube? You may want to check the BIAS POWER supply for consistent negative voltage in the range in the schemo."

The Super Reverb has only a pair of output tubes, not four. Of the 7581As, both showed gradually increasing grid emission, but one went a lot faster than the other. Since I'm not a total moron, a stable negative bias supply was one of the first things I checked :-)

"Also, 7581A usually biased hotter and put out more watts than 6L6GC. Mabe the extra current needed is revealing a parasitic problem, that the stock 6L6GC wouldn't reveal?"

I biased all the various pairs of tubes I used at the same level, around 35mA per tube. Yes, you should be able to run 7581As slightly harder, but in this situation, that would just have made them go out of control faster.

David

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