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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

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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

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    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
     

  4. HF1600ie

    HF1600ie Tele-Meister

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    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
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  5. HF1600ie

    HF1600ie Tele-Meister

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    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.

    :)
     
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  6. HF1600ie

    HF1600ie Tele-Meister

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    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
     
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  7. HF1600ie

    HF1600ie Tele-Meister

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    I can see some similarities in here between the VOX AC50CPH and old Marshall configurations.

    I was wondering if in the VOX AC50CPH, R25 is a fixed "5K" resistor where the Marshalls have the real "Presence" pot.

    Any thoughts on this ?

    VOX AC50CPH:

    upload_2017-9-18_14-55-39.png


    Marshall JMP50 :

    upload_2017-9-18_14-56-23.png

    Marshall JTM45 :

    upload_2017-9-18_14-59-5.png
     

  8. robrob

    robrob Poster Extraordinaire Ad Free Member

    Dec 29, 2012
    United States
    Yes, R25 is the 5k presence pot replacement.

    R62 limits the "volume" of the high freqs flowing through cap C49 to ground. C49 removes some highs from the NFB circuit which boosts highs at the speaker.
     

  9. HF1600ie

    HF1600ie Tele-Meister

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    Hi Rob,

    Got it ! See... i´m improving my schematic reading capacities :) Thanks to you !

    Seems like the Marshall use a ".1" cap, which should be 0.1uF which is 100nF. Same as the Vox.
    So, if C49 is removing highs from the NFB circuit, it means, less highs will "fight" against the other highs coming from the preamp, which will mean, more highs going into the speaker as you say.

    So, apart from R62, the only difference to the Marshalls would be a 5K pot in the place of R25.

    Does it mean that as it is the VOX is the same as the Marshall with the presence pot at zero ?

    First question : How could I place a real presence pot in there? The pot would have 3 lugs. One goes to ground... not sure about the others !!

    Second question : To emulate a Marshall-style presence pot at the "middle" position, can i simply change R25 from 4,7K to 2,5k ? Any other changes to consider ?
     
    Last edited: Sep 19, 2017

  10. HF1600ie

    HF1600ie Tele-Meister

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    Perhaps this should help.

    It looks like there´s no 27K NFB resistor. I can only see a 100K on the far right.

    A more detailed view of the PI :

    upload_2017-9-19_11-20-46.png
     

  11. robrob

    robrob Poster Extraordinaire Ad Free Member

    Dec 29, 2012
    United States
    You would have to remove R62 to equal the Marshall presence pot set to max bright. R62 "throttles" the cap down. The cap size sets the filter's corner frequency which is the same between the two amps but R62 reduces the filters efficiency--reduces the volume of the high freq bypass--it doesn't change the corner frequency.

    That 4.7k resistor labeled "on most amps" on the presence pot below is present in most original 5F6A Bassman amps. It was a production tweak that was never documented. It cuts the 4.7k pot resistance in half which reduces the presence pot's effectiveness, warms the bias of the LTP phase inverter and cuts negative feedback in half.

    The Bassman uses a 2 ohm secondary tap for NFB which has half the voltage output compared to the Marshall's 8 ohm tap. Marshall would have to use a 47k NFB resistor to get the same NFB as the Bassman (without the 4.7k resistor on the presence pot) and the Marshall 100k is the equivalent of the Bassman with the 4.7k resistor in place.

    To install a presence pot replace R25 with a 5k pot and wire it just like in the 5F6A here, but leave off the 4.7k resistor ("on most amps") on the pot:
    [​IMG]
     

  12. HF1600ie

    HF1600ie Tele-Meister

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    Rob, thanks for the explanation.

    So, If I am not wrong and read your comment correctly :

    1- R62 in the VOX is the same as the 4,7K resistor "on most amps" in the Fender ?

    2-With R62 in my VOX, and since I have R25 as well installed, does it mean I already have the typical Marshall presence pot at its middle position ? Because R62 is reducing the effectiveness of R25, the same way the "4,7K resistor on most amps" is doing to the 5F6A 5K presence linear pot ?

    3- Without R62 (removing or jumping?) I have the typical Marshall presence pot at maximum setting ?

    4- If I increase R62, it is the same as rotating the Marshall presence pot backwards (into zero) ?

    5- My Vox doesn´t have the 27K NFB resistor, does it ? If it does, where is it ? How does it compare to R62 when increased or decreased ?
    The Marshall JTM45 has a 27K NFB resistor and it seems to me the JMP50 has a 100K NFB resistor (displayed in the upper center part of this schematic), not down low as the others :

    upload_2017-9-19_14-41-3.png

    Cheers Rob
     
    Last edited: Sep 19, 2017

  13. robrob

    robrob Poster Extraordinaire Ad Free Member

    Dec 29, 2012
    United States
    1- No, the "on most amps" resistor is connected across the presence pot. Neither the VOX or Marshall circuits have an equivalent.

    2- R62 reduces the amount of brightness added by the presence cap. I can't say if it's close to the Marshall's presence control mid point though.

    3- Yes, jumper around R62 to equal Marshall at max bright presence control.

    4- It's not exactly the same but it's close.

    5- Your NFB resistor is off the schematic to the right. It's between the output transformer secondary and the presence control.
     

  14. HF1600ie

    HF1600ie Tele-Meister

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    1- OK

    2- Let´s say that removing R62 would be a typical Marshall presence set at maximum and 4,7K is reducing the highs. Then, probably a 2.2K will decrease the NFB and increase the presence effect, making the filtering of R25/C49 more efficient.

    3- OK. Usually at maximum, the sound is very very like a bell and bright. I like it up to a certain point.

    4- OK

    5 - I saw it. It´s a 100K resistor without a number just above the lettering "to 8ohm tap of the optx".
    I just read some people use a 47K in there if it corresponds to the 8ohm impedance. I have no idea of the impact in tone.

    Just read this : "I was running 100/8 and yup, it was gainier and looser. 47/8 was cleaner, crisper, and tighter. That is what I have stuck with. What can i say, I like a tight sound!"

    Hard to describe !
     
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