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Vox AC50 CPH - Presence knob - How does it work?

Discussion in 'Amp Tech Center' started by HF1600ie, Apr 4, 2017.

  1. HF1600ie

    HF1600ie Tele-Meister

    Age:
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    Sep 26, 2016
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    Hmm, so the Marshall presence control "preserves" those frequencies which otherwise would be removed, and sends them back as we turn the pot from 0 to 10. I have to study more about it.
     

  2. robrob

    robrob Poster Extraordinaire Ad Free Member

    Dec 29, 2012
    United States
    [​IMG]
    5k pot at bottom center is the presence pot.

    Yes. The presence pot has a cap to ground attached to the wiper. Move the wiper upward and high freqs bypass the resistor to ground instead of entering the phase inverter as NFB.
     

  3. HF1600ie

    HF1600ie Tele-Meister

    Age:
    36
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    Sep 26, 2016
    Portugal
    Then, i could say that this Vox has all the frequencies that the true presence pot would give while maxed? Placing this knob would give a darker sound if pot was at zero. I always thought that the presence knob was an "extra" when compared to the same circuit without it. Cheers
     

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  5. HF1600ie

    HF1600ie Tele-Meister

    Age:
    36
    116
    Sep 26, 2016
    Portugal
    Robrob,

    This weekend i´ve tried both the 500pf and 250pf - both 400/500v rated - around the volume pot of channel 2. One at a time.
    I soldered the capacitor directly to the pins of pot, via the back of the board.

    I have to say that 500pF seemed a bit harsh, more trebley harsh, probably because it went to mid-high territory and also, it seemed to me the bass was less warm a bit cut.

    On the other hand, 250pF gave a little "silk" to the sound and the bass didn´t seem to be affected. Maybe because only the real highs are "boosted" whereas the bass and mids remain balanced with each other.
    Even the fake "presence" pot, seems more integrated in the tone stack now.

    A proper high volume test will give me a better review, but I was playing loud enough at home I guess and kind of liked it.


    PS - My channel 2 gain pot also has a switch for two bright capacitors. But this effect, although probably based on the same principle, seemed to be more pronounced.

    I´m leaving the capacitor in there at the moment ! :)

    NEXT : The variable "slope" resistor - a contour knob. A fixed minimum of 10k will be placed and first I will use a pot to test. If I like it, I´ll place a rotary 4-positions in the back of the chassis, like this (15k-33k-56k-80k). This way I can guarantee position 2 is stock.
     
    Last edited: Apr 24, 2017
    robrob likes this.

  6. HF1600ie

    HF1600ie Tele-Meister

    Age:
    36
    116
    Sep 26, 2016
    Portugal
    OK, played last Sunday afternoon a bit at the backyard. Still, low volume, but the VOX AC50CP is sounding amazing. At a certain moment I found myself playing with the tone controls flat, or all flat with the treble control at 7. It seems I had room to either make it brighter or punchier if needed, or simply adapt it to a different guitar nicely.
    I even found myself lowering the presence knob below 5 to achieve some jazzy tones. The treble it adds/cuts is differently positioned in the EQ when compared to the treble knob ! :) It seems Presence knob now is useful to achieve thicker tones which somehow makes it useful.

    I could, however, for the sake of preciosity, change one of two capacitors to thicken the tone a tiny little bit. Either C28 - the plate coupling capacitor 1st gain stage v1a (from 2nf to 4,7nf which is stock again), or C33 - the plate coupling capacitor from the FAT switch valve - V2b, from 810pf to 1.5nf (stock is 22nf).
    I will probably go for C28. And that´s it. I don´t see what else I would need to change and the variable slope resistor will be just for fun.

    Played with different guitars. All sounded great.

    The 250pf mod on the volume pot is subtle, but has opened the sound just enough. That was a real nice one. I though that this treble bleed mods would only be used in the gain pots, usually.

    VOX should have sold this amp with this mods.

    :)
     
    robrob likes this.

  7. HF1600ie

    HF1600ie Tele-Meister

    Age:
    36
    116
    Sep 26, 2016
    Portugal
    By the way, I have forgotten to thank you all the support during the modification journey of my Vox AC50CPH. And also to other member - Lynxtrap - both have given me precious support !
    Cheers
     
    Lynxtrap likes this.

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