I got this really clean 1976 Mesa Mk I. Sounds way better then I expected. Anyway I want to make it sound better I test and compare and end up replacing the Chinese 12X7 tubes with Telefunkens and the 12AT7 with a black plate GE. I test the bias with the existing Chinese coke bottle 6L6GC and all the bias are right around 20 on a Webber bias right. I replace those with a pair of Fender labeled GE gray plates and a pair of GT labeled GE both sets of tubes at least twenty years old but test good. Bias for the new/old tubes is 36/21/21/31. Everything sounds fine as a mater of opinion it sounds better then it did when I started smooth, even, tight. Then I do this vibrato on the B string 14th fret with the gain boost pulled and the first volume on 7 the second volume on 3 and the master volume on 2. I get this nice feed back sustain thing just ringing sounds grand and the two tubes on the right start to glow like cherries no stop lights...WOW. Needless to say I shut the amp off immediately. I don't believe I hurt anything in the amp but can someone maybe tell me what happened?