For those of you with a lot of tube experience and expertise, I could use some advice. This is really the first time it seems I've got a tube go bad, and welcome any comments. Since the MC2301s are my first tube gear, this new territory, and it doesn't seem like there's a lot of commentary about trouble shooting or dealing with bad tubes. The system has been a dream since I bought it. I don't drive the volume levels very high with the rare and brief exception, and the amps are well-ventilated.
My left channel MC2301 went "pfffft" and the Sentry Monitor kicked in, turning the amp off, with the red indicator light blinking. So, it looks like there's a bad tube somewhere. Same thing happens after shutting down the amp and turning it back on, with the Sentry Monitor kicking in a few moments after the unit is turned on.
I have an Orange brand tube tester, and test all eight KT88s and the two 12AT7s, the Gold Lion tubes that I installed in August, 2022, ~18 months ago. All the tubes tested "Good" (for those of you with that tester it was all Good/8 or /9). My guess is there is about 1,500 hours on this set of Gold Lions. At this point I either have a tube problem that doesn't register, or something has gone bad with the amp.
I have the original tubes since they still have life in them, as well as another set of Gold Lions, so I'm able to test by swapping out tubes.
Swapped out the 12AT7s, and got the Sentry Monitor kicked in, so it looks like it's a KT88 or the amp.
Next, put the 18-month-old 12AT7s back in and swapped out the Gold Lion KT88s with the stock KT88s, and it is now working fine, and has run for about 45 minutes now, no problem, so it seems one (or more, however unlikely that is) of the GL KT88s is bad.
My next step will be to replace stock KT88s one-by-one with the Gold Lions until the Sentry Monitor turns it off again, to identify which tube is affected by an undetectable gremlin. I'll do this over the course of a couple days, as I turn the system on to listen to it, thus avoiding extra on/off cycles and wasting time waiting while tubes cool down.
Questions for those of you with more experience with tubes than me: Does this sound like the best approach? Is it "typical" if not common, that a tube could test as "good" but be a problem? Any other things to keep in mind?
I'll update this once I've resolved the issue.
