Quote Originally Posted by Opti-cal View Post
Thanks chaps,

just strange that it is perfectly silent in other preamps under the same conditions though.

Fairly sure its not microphonic either, I can tap on it less than gently with a screw driver and no sound through the speakers at all even on high gain.

Replacing the valve would surely cure the problem but I don't have another 12au7A grey plate spare . . . . Should have some clear-tops arriving soon though . . .
I'm not an expert but the voltage seen by the valve in one preamp is different to another. It is not microphony. Its a voltage induced oscillation in the tube which has a flaw.

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