Asher Guitars WD Music Products Amplified Parts Mod Kits DIY Nordstarnd Pickups Warmoth.com Reiland Pickups
Asher Guitars WD Music Products Amplified Parts Mod Kits DIY Nordstarnd Pickups Warmoth.com Reilander Pickups
Asher Guitars WD Music Products Amplified Parts Mod Kits DIY Nordstarnd Pickups Warmoth.com Reilander Pickups
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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 TDPRI Member

    Age:
    36
    84
    Sep 26, 2016
    Portugal
    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 TDPRI Member

    Age:
    36
    84
    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 TDPRI Member

    Age:
    36
    84
    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 at 5:01 AM
    robrob likes this.
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