Was messing around with new ideas for presence/depth/feedback when I saw the power tubes flash. After some consideration I came to the conclusion the PI tail had intermittent connection via the presence pot wiper. In other words the B line was switching through the PI down the nfb through the PT secondary to ground. That voltage transient of course being multiplied into the primary and the tubes. FLASH! I hope my OT is still alright...
Lesson: Never use only wiper to ensure dc connection of PI to ground.
PI tail causing flashing in the power tubes
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