I would get a new 6550 from C-J. Not sure if you have the GE or Svetlana, they now use Svetlana though. You should be able to isolate it to which monoblock first. That narrows it down to 4 output tubes, once you figure which mono it is coming from. Take the new tube, put it in place of one of the four, and fire the amp up. If it persists, put the original tube back in it's slot. Then repeat this procedure, inserting the new tube in place of the second original tube. Then , same for steps 3, and 4 if necessary. Hopefully, it's just a tube. Nice amps BTW. Keep us posted.
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