[TECHNICAL] Vox AC4TV Mods

Discussion in 'The Squawk Box' started by Robbks, Nov 23, 2016.

  1. Robbks

    Robbks Senior Member

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    Looking to Mod a Vox AC4TV Combo for a little better control of it's output volume for late night thrashing.. :D

    It's a pretty simple little amp, 12AX7 + EL84
    And I can see a pretty simple install if I'm Correct.

    Just adding a 1Meg Pot as Pictured (second Pic) at V3.

    anyone care to add advice, tips or have performed similar?

    Cheers
     

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    bilbarstow likes this.
  2. bilbarstow

    bilbarstow Premium Member

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    I'm pretty sure that would work. I have nearly the identical "MV" mod done to the Epiphone VJr that I rebuilt/modded from ground up (Turret Board build). That amp circuit design is very similar to the VJr Mods (Both are 12AX7 / single EL84 amps). If TWANG was still around, he could probably verify this for you.

    ETA: I just noticed the "attenuator network" portion of the drawing. Not sure how this pseudo-MV will play with that part ?
     
  3. Robbks

    Robbks Senior Member

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    Awesome,
    and a thread here about it
    http://www.mylespaul.com/forums/showthread.php?t=186850&page=10

    Essentially, I don't really like the built in attenuator on the output end.
    the 1Watt setting is OK, but I don't like the tone of the 1/4w

    I know that this will change the tone, but I'll have a lot finer control over it, aswell as still having the Att network to squash the output if required.
     
  4. bblooz

    bblooz Senior Member

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    I assume this would also work on the AC4c1, which is essentially the same amp without the attenuator.
     
  5. Robbks

    Robbks Senior Member

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    Speaking with my local guru of all things valve amplification last night (70yo electronics technician)
    All we need to make sure of is to choose an appropriate value Pot, (500k, 1meg) and change the resistor (R20, 220k in the schematic) to maintain the right Grid-Bias Resistance As we're now putting in a Resistor in Parallel with it, we WILL change the bias resistance and as such colour the tone with it.

    So it may be a bit of trial and error to work out the sweet spot of actual input (to the pre-amp) volume and tone.

    My theory is to have the new Master Volume set at 12 o'clock with an Audio Taper pot
    And that be the "Standard" total bias resistance of 220k and that match the Standard, factory tone of the amp.
    Above and below that volume, some slight changes in tone will just have to be dealt with.

    This is all theory of course, in reality it may not even be detectable.
     
  6. Paragon

    Paragon Senior Member

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