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
Join TDPRI Today

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:
    36
    117
    Sep 26, 2016
    Portugal
    Hi there,
    I don´t understand so well how this Presence control works. It is not the typical Presence control as in a Marshall which adds that kind of bell-like treble as the Master volume goes up. Instead, it seems to be more of a cut control, which means that at 10, all the natural treble is going through the signal. Otherwise it is cutting.

    What´s the influence of C6 and C7? What´s R4 for? Maybe R4 is to set a minimum amount in the signal between C6 and C7 ?

    Thanks everyone!




    upload_2017-4-4_10-56-13.png
     
    Last edited: Apr 4, 2017

  2. HF1600ie

    HF1600ie Tele-Meister

    Age:
    36
    117
    Sep 26, 2016
    Portugal
    Ok, I can be wrong, but maybe C6+R4 are a treble bypass circuit, just like those on a guitar´s volume pot?
    Some use the resistor in parallel with the capacitor, others use it in series.
    In some cases, there´s no resistor. Here we have it in parallel.

    So, decreasing C6 will retain more treble, by shaving off bass. Is this right?

    For C7, it goes to ground. I don´t understand it. Maybe like a guitar´s tone pot ?

    So, when set at 0, the "presence" knob is cutting treble. When set at "10", the "presence" knob is cutting bass ?

    While using it in reality, it seems to me that when set at 10, there´s a bit more cutting edge to the sound and anything below 5 is somehow muffled.

    Any comments?
     

  3. robrob

    robrob Poster Extraordinaire Ad Free Member

    Dec 29, 2012
    United States
    It's just a funky RC low pass treble bleed, the presence pot allows c6 and r4 to be bypassed straight to c7 as you turn down the pot. R63 is the R in the RC filter, c6 & c7 are the C. R4 reduces the effectiveness of the RC filter. Turning up the presence control puts c6 and r4 into the circuit which cuts the effectiveness of the treble bleed RC filter making the tone brighter.

    The main reason a presence or resonance control is valuable is its location in the amp circuit. They are late controls that affect only the phase inverter and power tubes so you can tweak the tone without affecting the substance of the tone, especially the overdrive tone.

    This gimmick "presence" control is right at the tone stack and volume control so it's almost worthless.
     
    HF1600ie and JD0x0 like this.

  4. Forum Sponsor Sponsored posting

  5. HF1600ie

    HF1600ie Tele-Meister

    Age:
    36
    117
    Sep 26, 2016
    Portugal
    Ok. Thank you! The treble it seems to "add" while maxed is different from the normal treble pot. It seems to either cut or add at a higher frequency and it has a short of peaky instead of wider "Q". It adds or cuts natural briliance. So how can i have more "presence" when it is maxed? Also, how to make it cut less treble when rolled back? Below half it is not that useful. If i increase or decrease each component, what can i expect? Just trying to make it useful and more powerful....because it is a shame it is not a real presence pot!!!
     
    Last edited: Apr 4, 2017

  6. HF1600ie

    HF1600ie Tele-Meister

    Age:
    36
    117
    Sep 26, 2016
    Portugal
    Wait a minute... is this like a guitar´s tone pot with the capacitor (to cut high frequencies) going from lug 3 to the back of the pot which is then is connected to ground?

    At the same time, a treble bleed capacitor in parallel with a resistor are connected the traditional way between lugs 1 and 2 ?

    If so, please tell me if I am wrong or right :

    - To increase maximum treble response when Presence knob is maxed : Decrease C6 value.

    - To decrease maximum treble cut when Presence knob is rolled back : ??? 470pf is already too low. I am think about the same affect as a guitar tone pot....but usually the values are from 10nf to 47nf.

    Maybe I´m confusing... any help is appreciated !
     

  7. HF1600ie

    HF1600ie Tele-Meister

    Age:
    36
    117
    Sep 26, 2016
    Portugal

    Attached Files:


  8. HF1600ie

    HF1600ie Tele-Meister

    Age:
    36
    117
    Sep 26, 2016
    Portugal

  9. HF1600ie

    HF1600ie Tele-Meister

    Age:
    36
    117
    Sep 26, 2016
    Portugal
    Any blessed soul could help me with the above two diagrams ?

    Please.

    Thank you so much.
     

  10. AAT65

    AAT65 Tele-Afflicted

    Age:
    52
    May 29, 2016
    Edinburgh, Scotland

  11. HF1600ie

    HF1600ie Tele-Meister

    Age:
    36
    117
    Sep 26, 2016
    Portugal
    Yes, but I am unsure how those two figures above in the topic compare to my "presence" knob.
    And also, how it will react when the components are increased or decreased.

    Also, If I can convert the first diagram into the second, swapping the parts. :/
     

  12. robrob

    robrob Poster Extraordinaire Ad Free Member

    Dec 29, 2012
    United States
    I had to re-draw it to better understand it:
    [​IMG]

    The Volume pot is a standard volume control.

    I don't understand how C6 and R4 can do anything because they are bypassed by the yellow diagonal wire. The setting of the Presence pot does not change the bypass situation. Perhaps the schematic is wrong?

    Turning the Presence pot max down (counter-clockwise) bypasses all pot resistance. R7 + C7 form an RC low pass filter with a cutoff frequency of 605Hz. Only frequencies below 605Hz pass out of the volume pot.

    [​IMG]

    Turning the Presence pot max (clockwise) adds resistance to the top of C7, reducing the effectiveness of the low pass filter which adds high frequencies back into the mix.

    [​IMG]
     
    Last edited: Apr 6, 2017

  13. robrob

    robrob Poster Extraordinaire Ad Free Member

    Dec 29, 2012
    United States
    HF, if your presence circuit really matches the schematic (which is doubtful) then C7 is the only component to adjust. Increasing its size will lower the cutoff frequency and give the presence control more mid and mid-low authority. Decreasing it will force the presence control's focus onto higher frequencies.
     

  14. HF1600ie

    HF1600ie Tele-Meister

    Age:
    36
    117
    Sep 26, 2016
    Portugal
    Dear Robrob,
    Thank you so much for such a detailed explanation.
    So, assuming the schematic is correct (which we don´t know), your analysis may conclude that :

    1 - My presence knob is just a "normal" tone cut (low pass filter) formed by VR5 (500K pot) + C7 (470pf). And that´s it ? Just like a guitar tone pot?

    2 - When at minimum it will be sending high-frequencies to ground influenced by C7.

    3- When at maximum, it is not cutting any high-frequencies anymore and just let´s the normal signal pass through, getting back the high frequencies.

    4- When at maximum, C6+R4 are bypassed. These, and assuming schematic is correct, won´t have any impact at all on the signal.

    5 - C6 + R4 could be removed without any impact on the overall effect, either at maximum ou minimum setting of Presence knob?

    6 - It is not possible to add more high frequencies with this "presence" control when it is maxed. At least, no more than the ones already existent on the signal path. This "presence" control is only able to cut high- frequencies.

    7 - It is possible to decrease the maximum cutting effect, by decreasing C7. Less cutting will be applied when rolled back.

    8 - Any modification I could do to have a more interesting "presence knob" ?

    9 - Either "the specialist" or "bitmo" single knob tone controls would work better? Is it possible to have a conversion?

    Sorry for all these questions, just trying to understand deeper my AC50CPH (the ugly duck) :) actually now becoming beautiful...
     
    Last edited: Apr 7, 2017

  15. HF1600ie

    HF1600ie Tele-Meister

    Age:
    36
    117
    Sep 26, 2016
    Portugal
    I´m trying to see if the schematic is correct. Just found these pictures. Presence is VR5.

    Looking at the front is the 3rd pot from left to right.

    Looking from the back it is the 3rd pot from right to left.

    C6+R4 seem to be in place, don´t they?
     

    Attached Files:


  16. robrob

    robrob Poster Extraordinaire Ad Free Member

    Dec 29, 2012
    United States
    As far as I can tell the PCB is wired like the schematic.

    [​IMG]
     

  17. HF1600ie

    HF1600ie Tele-Meister

    Age:
    36
    117
    Sep 26, 2016
    Portugal
    Yes!! It also seems the same to me regarding the presence knob.

    Can you please help me clarifying the questions of the previous post (before the pictures).
     

  18. robrob

    robrob Poster Extraordinaire Ad Free Member

    Dec 29, 2012
    United States
    Last edited: Apr 7, 2017

  19. HF1600ie

    HF1600ie Tele-Meister

    Age:
    36
    117
    Sep 26, 2016
    Portugal
    Yes, usually late. I think i´ve seen it after the PI.

    But as you can see in Channel 1 (Tone cut) and Channel 2 (Presence), these two are in the tone stack. I can be wrong, but both seem to be equal Low-Pass filters.
    However, I have to say that in Channel 1, the "tone cut", which works counter-clockwise (no cut at zero), works remarkably well. So simple and good for very quick adjustments in the upper highs.

    The "Presence" in channel 2, well, not so well, specially below 5. The sound becomes very very muddy and with a light fizz in the highs. Go figure. Not a great combo, I have to mod it somehow. It works well, between 5 and 10. Suggestions ?
     
    Last edited: Apr 7, 2017

  20. HF1600ie

    HF1600ie Tele-Meister

    Age:
    36
    117
    Sep 26, 2016
    Portugal
    Both channels tone stack : 1 and 2
     

    Attached Files:


  21. robrob

    robrob Poster Extraordinaire Ad Free Member

    Dec 29, 2012
    United States
    Looking at HF's schematic it looks like Vox made a late tone circuit production change by adding in a bypass trace around C6 and R4. So yes, C6 and R4 are bypassed and have no effect on the amp's tone.
     
    Last edited: Apr 7, 2017

IMPORTANT: Treat everyone here with respect, no matter how difficult!
No sex, drug, political, religion or hate discussion permitted here.