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hum-loop-like problem

Discussion in 'Electronic Repair' started by N_Cook, Sep 29, 2011.

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  1. N_Cook

    N_Cook Guest

    Laney VC30 valve amp, works fine in its own accord. But owner wants to set
    it up with another amp , another make and solid state if relevant, for a
    pseudo stereo set up using the A and B outputs of a pedal effects unit.
    Produces mains hum on switching from standby to on. Making one of the feeds
    isolated and relying on the grounds from each amp makes no difference, still
    intrusive hum.
    Bridging stout wire from mains earth to the VC30 input socket makes no
    difference .
    Pulling V1 from the amp still induces hum in the other amp. Had to return
    the amp for owner to do a gig as normal mono set up. I did not try an
    isolation transformer and I did not get inside the works, just testing the
    valves. I'm assuming internal shared ground path for HT return current ,
    anyone any thoughts for the next time if I'm faced with this.
    Only a year old so maybe PbF problem at the input perhaps
  2. N_Cook

    N_Cook Guest

    less than 0.1 ohm between input ground and mains ground in all 3 cases of
    the VC30 , his other amp and my amp to confirm a VC30 problem
  3. josephkk

    josephkk Guest

    Is one side of line bonded to ground where you are? If so there may be an
    incorrect bonding or lack thereof.

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