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Anyone ever experience a very low freq. ocscillation with an amp when it is at idle and atop the speaker cab? When it first occured, I thought it was my basement's sump pump running. Not really the putt putt of motorboating, more like a low pitched whirring sound. If I move it off the cab or even pick up one end of the head it stops. It is not very loud but its annoying. The amp sounds fine other than that. This is a early S.F. with the AB763 circuit.
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