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Sunn 1200s with mid boost hum problem. Help!.................

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  • Sunn 1200s with mid boost hum problem. Help!.................

    Hello all, really hope somebody can be of help, this amp's driving me mad.
    Basically, when you engage the mid-boost switch, there's a really loud hum. I've had a look with a probe and it appears around the 6AN8, at the anode and cathode of both sides of the valve. I've tried a new 6AN8 and also tried hard-wiring the switch in case it was faulty (I've also replaced all the filter caps in the power supply as a matter of course). None of the caps around the valve look to be leaking, could it be something to do with the feedback?
    Any ideas?
    Cheers, H.
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