I feel compelled to add my response here . . . Hooray to Jim McShane for going above and beyond on replacing questionable tubes, even when the problem isn't the tube.
I just read his old FAQ and agree with the whole thing, and I agree with him that I get the same questions over and over and over and over.
5-10 years ago we didn't have this problem. People with tube amps were both electronics enthusiasts and audio enthusiasts. Now 90% of them don't know anything about electronics, and unfortunately that a huge negative problem if you want to have a good experience with a tube amplifier.
99% of the time, tubes go bad for one of two reasons
1-operator error (incorrect or no bias adjustment, turning the wrong pot, turning it the wrong way, not knowing how to use a meter, etc.)
2- loose tube socket pins
1% of the time it's just poor quality control and especially because in these modern times, tubes aren't made the way the were 50 years ago.
50 years ago, tubes were all hand-made and hand tested, and they were built to last forever. These days, tubes are a commodity, made by someone who
purchased a license and some old equipment from someone who went out of business, to make a quick profit, and too bad if they don't make it thru a 90 day warranty!
Even just a couple years ago, I purchased 4 new EH tubes (now I hate to say EH tubes are bad, they aren't, but you get what you pay for, and they are OK for the money).
Four new KT88s. 3 of them wouldn't work in my ST70 amp. Runaway bias. The cure for runaway bias I've found to be lowering the grid resistance to ground.
Used to be 270K was fine, then it was 220K. I haven't found an EH KT88 yet that will hold bias unless that resistance is only 150K or less, so for the past 3 years all the VTA70 kits I sent out now use 150K grid resistors to ground. I've heard many people on other forums recommending that change to 100K.
I've never tried a KT120 but I see Jim's FAQ states that it wants LESS than 51K. That's simply unbelievable (well, not really, but it's kind of ridiculous, since I've never seen any tube amp use less than 100K).
So Bob, if you're reading this, I'd recommend you make a 100K resistor standard in all your bigger amps (especially the M125s) and add a note that if you're going to try using KT120s to change that to a 51K resistor.
I think 90% plus of the people on this forum are very well educated, now that they have this experience with tube amps.
But for newbies - take note!! At least learn how to use a meter, understand tube bias, and make sure the tube fits tight in the socket pins.
THEN - enjoy the music!!