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VOX AC30CC2 combo amp

Discussion in 'Electronic Repair' started by Phil Allison, Jul 28, 2013.

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  1. Phil Allison

    Phil Allison Guest

    ** Hi,

    the " VOX AC30CC2" is made in China and designed by Korg of Japan. The two
    12 inch speakers say made by Wharfedale - but that too has been a Chinese
    brand for a long time now.

    The only European parts in the whole thing are the Russian made valves, Korg
    must have developed have a low opinion of Chinese ones. No surprise.

    Some of the many oddities include using multiple op-amps in parallel for
    reverb drive and 500V, TO92 mosfets acting like small triodes for the
    tremolo phase-shift oscillator.

    Plus - the amp is exquisitely engineered to destroy EL84s and GZ34
    rectifiers in short order.

    There is a STANDBY switch that lifts the CT on the secondary of the power
    tranny - but in my example a 39kohm 5W resistor has been added across switch
    by the makers. It's not in the schem I found on-line.

    The ONLY thing the resistor does is make the amp hum when in standby mode
    !!!!!

    Takes about 10 seconds after you move the toggle for an annoying hum to
    appear - by this time the main HT has dropped to 20V.

    Things that stop the hum:

    1. Remove the 39 kohm.

    2. Remove the GZ34.

    3. Ground any of the grids of the EL84s.

    Removing the phase splitter valve has no effect.

    Variacing the AC down reduces the hum in proportion.

    Barking mad.



    .... Phil
     
  2. Phil Allison

    Phil Allison Guest

    "Gareth Magennis"

    ** Max points to you for asking good questions.

    Not one, but numerous design errors ( never seen in a British VOX amp) are
    involved.

    1. Destroying the EL84s.

    The Korg AC30CC operates EL84s beyond their safe dissipation limits - with
    a DC supply of 320V and a cathode current of up to 60mA each at idle, the
    poor valves simply cook.

    PLUS there is a BIAS switch on the back panel labelled "HOT"and "WARM".
    Hot = 50 ohms common cathode resistor, warm = 82ohms - both are too low,
    so the valves fry.

    Then there is the tremolo circuit which drives all four EL84 grids with a
    sub sonic sine wave to modulate gain - a crude idea not seen since the 60s
    in a few low powered Fender models. It kills valves.

    2. Destroying the GZ34.

    The standby switch does that - by creating massive surges at each switch on,
    well beyond the safe limit.

    British AC30s, with a GZ34, simply never had such a switch.

    I have read on VOX forums that the 39kohms is a kind of fuckwit solution to
    the problem dreamt up by the Chinks.

    Well, it aint.

    It just makes the amp hum.

    BTW:

    There is a simple fix for the EL84s overheating.

    I do it routinely, but never tell owners.



    .... Phil
     
  3. Phil Allison

    Phil Allison Guest

    "Arfa Daily"

    ** Sovtek is the trading name of a US based valve importer - they make
    nothing.

    The "New Sensor Corporation" own that name and Mullard too.

    They could print either on dog's turds if they wished.



    ..... Phil
     
  4. Phil Allison

    Phil Allison Guest

    "Gareth Magennis"

    ** First I replace the cathode resistor with one of about 120 ohms, 5 or
    10W.

    The cathode voltage rises a bit at idle and quite a bit more when the amp is
    driven to clipping - resulting in some crossover like distortion and loss
    of power output.

    Next I add a couple of 5W zeners in series across the 120 ohm - picked so
    they barely draw current at idle but clamp the voltage at about 13 or 14V.
    So we now have *fixed bias* done at the cathodes.

    The result is each EL84 now dissipates a comfortable 9W at idle.

    The output power *increases* to over 30W with no sign of crossover.




    ..... Phil
     
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