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My marshall valvestate 100w combo makes a high pitched noise, similar to feedback but not, almost immediately after switching on. Is it maybe that the preamp valve needs replacing? If you tap the amp it usually increases, the only way to stop it is to turn it off. It's sometimes not too bad and playable, but always eventually starts to whine increasingly loudly.

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One of the tubes COULD be microphonic.

Take back off and use a pencil to lightly tap each tube to see if one causes noise.

Posted on Sep 10, 2009

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