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I have a phono preamp that gets super microphonic and noisy for like a week at a time and then back to normal for months. If I tap a tube or the case or the interconnect it's attached to with an eraser the thump is huge and then nothing when it's not happening. This seems weird.
I had a driver tube that started that way. One or two days it was fine, then the next few days it would get buzzy. Finally it would just start to buzz after 10-15 minutes after it warmed up. A gentle tap would stop the buzz for a minute or two.
What I believe was happening was an internal weld was failing.
Time to replace the microphonic tube.
Why intermittent ? Maybe some support is nearly fixed, when it warms up
it might or it might not slip a bit. If it don't slip then it's tight,
if it slips then it can move with any vibration.
Replace tube !
things expand when heated and inside tubes it gets very hot. Super hot in fact in order to get the electrons to flow freely off the cathode material
occasional tapping may be necessary until you can find better tubes.
Hey, it happens, don't lose sleep.
*Freudian slumber, plenty of sound
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