Tube microphonics in a preamp


I just installed some Amperex 7308 white labels into my Blue Circle BC 21.1 preamp. They sounded wonderful right off the bat but exibited some microphonic (tinging and ringing) through the speakers when I changed the volume or even touched the surface of the unit. THis was not apparent on listening to music. Now that they have had about 30 hrs on them, the microphonics are gone. Normal phenomenon? Have the tubes gone bad now or have they broken in. I don't notice a big degradation in sound quality...I've probably gotten used to them. Would YOU keep using them? Thanks. Dave.

I've experienced the same ringing problem with a couple of my Amperex 6922's from the preamp section over a period of time but decided to remove and replace the microphonic one...the "howling" was just too unbearable.

It's peculiar that the "tinging & ringing" from your tube(s) just went away...I would have them checked & tested for leaks just to be certain.
No phenomenon here.
There maybe bad solder joints or poor contact at tube sockets that needs to be cleaned once-a-while.
Ringing will go away after the tube has been hot and the parts have expanded. I don't know why you would think they went bad.

Or the reverse can happen, and they go microphonic. Don't cycle them on/off. Bad for them. Don't tap on will loosen the parts and make them microphonic in some cases.

Don't put crap in your tube sockets to condition. Don't pull the tubes in and loosens the sockets. Tube sockets are designed to scrape the pins and make contact.