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VOX AD30VT 'valve/tube' amp

Discussion in 'Electronic Repair' started by n cook, Apr 13, 2006.

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  1. n cook

    n cook Guest

    Retro look of the 1960s Vox AC30 amp made probably 2004 with lots of SM
    digital chippery for effects undreamt of in the 60s.
    One 12AX7 , but is it for show only ?
    Heaters have an AC supply but the anodes don't seem have any HV components
    attached to them. The speaker o/p is a non-retro National Semi LM3886T.
    I've not powered up yet as only poking around cold at the moment as I've
    never seen the like before, all seems DVM ok for power supplies etc.
    Apparently went pop between songs and then no more.
  2. Arfa Daily

    Arfa Daily Guest

    I had one of these in for repair a while back. As far as I recall, the
    triode stage is genuine, but not run from a particularly high HT voltage.
    You can get away with some quite low anode voltages on these twin triodes.
    Elektor mag have published a few circuits in recent years, where they have
    run these at low HT. The digital simulations of various retro amp types, was
    actually remarkably good.

  3. n cook

    n cook Guest

    I'll get around to powering up tomorrow but from poking around there are no
    capacitors with greater than 50V dc working anywhere on the ps and o/p
    board, I've not undone the digital effects board.
    I've just noticed a 3.3V v. reg.
  4. n cook

    n cook Guest

    Damn hybrid O/P devices , -V rail to o/p short on the LM3886T.
    Not being abused and no audio troughput at all when it failed.
    Is there a learned file on WWW on why these sort of devices fail ,
    a loudspeaker wire short or heavy throughput you could understand.

    So ten squid to replace rather than just a 3055 or 3773
  5. n cook

    n cook Guest

    I will probably use a 100W TDA7294 as I've a couple at hand, pinning swapped
    to match the 68W LM3886.
    Monitoring phones outlet, all effects and classic amp colourations are
    functioning so the valve is presumably an expensive and highly inefficient
    indicator, for show only, the maximum DC on any of its pins wrt ground
    is -15V
  6. default

    default Guest

    No experience with the VOX amp, but in the 50's I do remember a
    pop'tronics article on using tubes "starved for voltage" They had a
    radio with a plate supply of +12 volts driving a single transistor
    class A transformer coupled output.

    I don't remember which tubes they used, but they were standard 6 & 12V
    filament parts.
  7. I seem to remember the was PC motherboard with a 12A?7 on it a few
    years ago -for that authentic valve sound -
    car radios with valve front ends and no vibrator pack, but, they were
    special valves with an extra grid I think.

  8. That motherboard was made by Aopen, which is Acer computers.
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