Microphonic tube vs. fillament rattle

[Axtman]

I know that a tube is microphonic if you tap it and the amplifier starts feedback howl.

What about a tube that has a high pitched "tink" sound or a lower pitch "bvvvt" sound when you tap on it yet does not feedback? What about a tube that you can hear sounds when you rub your fingers on it?

Should a tube be dead quiet in an amp that is cranked (for testing purposes)?

How do I know what tube to keep and what to throw out and replace?

[stokes]

All tubes are microphonic to some degree.Some preamp tubes will squeel like hell in a high gain amp,but work just fine in a lower gain amp.Its just the nature of the beast.How do you know what tube to throw out? When it becomes problematic in your amp.

[Enzo]

Feedback describes the situation where ENOUGH signal from the speaker is picked up by the microphonic element to sustain oscillation. but in situations where there is less than that level, the tubes can still be microphonic.

COnsider a microphone on your stage. Turn it up enough, it feeds back, turn it up less it doesn't. it is still microphonic either way.

And I can;t think of any better way to put it than stokes did, as to when to discard a tube.