Sometimes placing the offending valve in a diffrent position in the preamp ie; a lower gain section can remedy the problem, without knowing the preamp your using, i have no idea if this is posible.
A...
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 . . .