I was listening to some music last night and several times I thought I heard something that didn't sound quite right: A high-pitched ringing, not constant, but coming and going. It didn't seem related to anything in the music (in other words, it didn't happen on drum beats, cymbal crashes, or anything in particular, just random). After 15 minutes or so, I finally managed to pause the CD while I was hearing the noise and convince myself that it I really was hearing it. The tubes had been on for about 30 minutes, so they should have been fully warmed-up.
Does this mean I have a tube going south? Or is this a form of microphonics? Being relatively new to tube gear, I don't know how they usually fail. Anyway I switched them out and it seems okay now. I just thought I'd ask. A ringing like that seems like it could be hard on tweeters, if it happened to occur when the volume was up. Last night I was listening at relatively low levels.
My gear, if it matters, consists of a modified Tosh 3950, Dodd ELP (tubes were JAN 6922 Phillips) Sunfire 300 amp and SRS2's.