Quote Originally Posted by farflungstar View Post
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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Yup, that sounds about right, I suspected it was just a mis-match between pre-amp and tube. A real shame as 85% of the time it sounds top notch . . .

Probably worth searching out a 12au7A which does not whine. The one on the other channel is fine!