I have a Blackheart Little Giant loaded with Sylvania old school tubes. I recently had to change out the power tube, cause the output dimished greatly in either pentode or triode mode. Put another Sylvania in there, sounds great. Out of curiosity, I brought to my local tube amp pro, and he tested it for me. It tested fine, everything within range. So what they hell happened to it? I don't think there is any thing wrong with the amp, as another tube sounds fine.

Pain is temporary. Stupid is forever. - Some really friggin' smart dead dude
If the tube is good then maybe it just needed to be biased?

I think this might apply to your situation...

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