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Vox AC30 6TB, 2002

Discussion in 'Electronic Repair' started by N_Cook, Jan 14, 2009.

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  1. N_Cook

    N_Cook Guest

    Normal or abnormal ?
    Came in for an intermittant problem, fixed, and the following the owner
    lives with but is it normal or abnormal?
    With feed into a "normal" input and the "brilliant volume" at zero then
    there is no control of the bass on the tone control, you need to have a
    little advance on the brilliant volume.
    Trouble around V7 ECC83 ?
    DC voltages cathode,grid,anode
    "brilliant half" 0,1.4V, 182V
    "tone half" 182,183,290V
    all associated pots ,Rs and Cs check out agreeing with schematic as does the
    valve and same Vs with a different valve
  2. Geo

    Geo Guest

    Assuming I am looking at the correct circuit - the bass control is in the output
    of the brilliant circuit only. The "brilliant" and "normal" are mixed at the
    junction of the 220k resistors so the bass control will have no effect on the
    normal channel.
    The brilliant volume control effectively grounds the grid of V7a so I would
    expect 0v there and 1.5v ish on the cathode.
  3. N_Cook

    N_Cook Guest

    So normal, in summary, for this amp?
    The "brilliant" half is upper in this section of schematic

    There is treble control but not bass control with "normal" input and zero on
    brill vol.
  4. Geo

    Geo Guest

    Yes - that is the circuit I have.
    Unless there is signal (e.g. from guitar) through the brilliant circuit then
    neither bass nor treble should really have any effect.
    The position of the brilliant volume control should not affect the normal path.
    Is it possible that C10 - the common cathode decoupling of the first valve is
    failing and letting some signal through from the normal to brilliant section?
  5. Eeyore

    Eeyore Guest

    The modern 'copies' are shit.

    I have a file full of AC30 schematics fronm the earliest to quite new. Will that
    help ?

    Often, guitarists compare 'their' AC30 with an well-maintained original and
    complain it sounds shit. They're right. It does but there's nothing wrong with
    theirs it other than cheapskating.

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