Is a loose mica flake an issue in a 12AT7? It was purchased from Upscale Audio as 'Kevin's Stash' grade. I emailed their customer service and they said it was not an issue, but I would like a second opinion. It was purchased as part of a backup set, so I haven't tested it as its a PITA to pull out my preamp.
Mica is used as a thermal and electrical insulator so the stray loose piece will not impact the performance of the tube so you should be fine
HRm, let's take a closer look: Were it just a lightweight piece of non-conductive chaff that made it's way into the tube during manufacturing, perhaps of no consequence. You can't see anywhere that it has broken off, that it wouldn't instead be shipping damage? But then you have it bouncing around in there during shipping and what looks like very lightweight connecting wires between the pins and the tube internals that it could break. Perhaps even something else loose that doesn't belong, which I've pointed arrows at. I'd roll it around and see that indeed this is all that is loose, there's no filaments dangling. Then install. Could be the best sounding tube yet, or could be sparks.
Looks like it mechanically supported part of the internal structure. With it now missing from its place, is there a chance it is, or could become, microphonic?
Loose mica or any wear in a mica is a bye-bye. First microphonics in the few weeks or months then will follow with a tube short. I had so much bad luck with my favorite tube being consumed like that. I started using a tube damper which I hate to do for its suppresses a sound that I really like. But so far its been a year and a half now and I'm not getting that symptom anymore even using the same batch of tube.
Actually, it isn't so straight-forward because many tube shorts are high impedance and/or only reveal themselves when high voltages are present.
As long as the mica flake doesn't fall inside of the plate assembly, it's no big deal at all. Mica is very light weight and it's not going to bash its way through the internal wires at the base of the tube. Plus, it is a superb insulator, so it's not going to short anything either.
Uncle Kevvy charged you a premium. I would not want to use it. I wouldn't test it in your preamp, that's not a good tube tester. They should have spotted that , especially if they hold the Stash tubes to such high standards. I have had a few of Kevin's Stash and they were good, I would not accept that as the best of the best.
