It's all this worry and messing around that had me gradually switch to modern tubes nearly entirely. One example: I put a pair of NOS RCA Black Plate 6V6's in my Vintage 65 BFDR. That amp runs 460-465 plate volts commonly. They matched well but were very high reading ma when I installed them. (like 55 ma!) So I made the bias adjustments and the amp sounded fantastic. Forward a few days and we have a gig. About half way through the first set the amp pops/flashes and some smoke comes out of the back. Shorted power tube .....and it destroyed my original output transformer!