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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

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    What do you mean with "late tone circuit production change by adding in a bypass trace around C6 and R4" ? That they made a mistake without knowing it ?

    Maybe C6 and R4 and only useful while rolling back the pot ! The moment you start rolling back, they enter the path.
    Probably they create a different shapping to the frequency when the treble is starting to be cut.

    Maybe a good mod would be changing R4, so that the effect of this knob while cutting treble would be less dramatic and still, the frequency cut point would be mantained.
     
    Last edited: Apr 7, 2017
  2. robrob

    robrob Poster Extraordinaire Ad Free Member

    Dec 29, 2012
    United States
    I'm thinking they decided to tweak the voice of the channel and decided to bypass those two components to take them out of the presence circuit. Simple production change but it does seem like they'd save some cash and not install the two bypassed components. It might just be a plain old mistake.

    Nope, they're bypassed no matter the pot setting.
     
  3. HF1600ie

    HF1600ie TDPRI Member

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    If they´re bypassed no matter what the pot setting... argh, then it´s just a simple tone cut knob. I kind of like it in Channel 1 (even if it is in the tone stack) , but not that much in 2.

    At first, by looking at the schematic, it seemed to me like a high pass filter, as you can see in guitars.

    Is there any way that I can make the cutting less effective? I mean, the cut off point would be mantained, but the dry vs wet signal would let the dry signal stronger. So it would cut treble, but with a bit less effectiveness.

    Cheers
     
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  5. robrob

    robrob Poster Extraordinaire Ad Free Member

    Dec 29, 2012
    United States
    You can change the presence controls cutoff frequency by altering the size of C7.

    You can reduce the filter's effectiveness by adding resistance to either side of the presence pot. More resistance = less filter action.
     
  6. HF1600ie

    HF1600ie TDPRI Member

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    For instance, increasing R4 = less effectiveness. that would cut less highs. It´s an option.

    But when I think of it, I don´t even know if it´s worth the hassle. I wish I could do something GOOD with that Presence control.

    Is there anything simple you could suggest and that could be worth modding ? (apart from leaving it the way it is) :) Maybe transforming it in a "Contour" kind of thing or a real presence knob?
     
    Last edited: Apr 10, 2017
  7. robrob

    robrob Poster Extraordinaire Ad Free Member

    Dec 29, 2012
    United States
    R4 is bypassed so changing it won't alter the tone.

    Do you have the full amp schematic? I couldn't fine it online.
     
    HF1600ie likes this.
  8. HF1600ie

    HF1600ie TDPRI Member

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    Hi, in that case, I think I will definetely change that presence control to something else.
    Here is it (only available the 100W version). Mine is 50w.
    http://bmamps.com/Schematics/vox/Vox_AC100CPH.pdf

    I´ve made those mods in the preamp section so far. Seems way more "balanced" than it came from factory. The voltage dividers are now stock again at 470k.

    I can´t get more high end without sacrificing the low end, and vice-versa. I think it should retain more highs when I roll back my guitar volume pot, as it becomes a bit dark. But that means I need to have plenty of gain in those high frequencies, I don´t know, like a Bassman59. It is a trebly amp, but not all treble is equal :).
    The same way it is a warm amp, but not all warmth is equal.
    Doesn´t have much compression. The sound is raw and stiff !!

    The real tone only comes after pushing it after 4 in the volume and master volume.
     

    Attached Files:

    Last edited: Apr 10, 2017
  9. HF1600ie

    HF1600ie TDPRI Member

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    Hi Robrob. Did you
    have the chance to look at it?
     
  10. robrob

    robrob Poster Extraordinaire Ad Free Member

    Dec 29, 2012
    United States
    HF, I can't read that schematic's component values. Normally you would increase the coupling cap and preamp bypass cap size to fatten up the bottom end. Too much bottom end turns to mud during heavy overdrive though and the extra power (voltage and amperage) in bass freqs can cause blocking distortion.

    An added or increased size bright cap around the volume controls will help keep the amp from getting too dark at lower volume.

    More here: https://robrobinette.com/Voicing_an_Amp.htm
     
  11. HF1600ie

    HF1600ie TDPRI Member

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    Hi Rob,
    It´s strange, the link is broken.
    Here is a copy of it :

    https://drive.google.com/open?id=0B7QqUhYAtDF9anJJZ09ZYUdxME0

    The amp already has a bright capacitor around the volume pots. At least on Channel 2. It is the "bright" switch.

    I cant decrease the cathode bypass caps to 4,7uF or 1uF. The sound will be great at lower volume, but will be cold when playing louder, so I placed 330uF (same as nothing). For the coupling capacitor (C28) on the plate - 2nF is the minimum.
    High frequencies will come up at a great expense on the bottom end which will become tiny as the volume goes up.

    I´ve already made some mods as you can see in previous post. The mods that had the greatest impact are in C33 (22nf to 810pf) and R44 (4,7k to 12K). These should only affect the Fat switch which seems to be an extra gain stage on/off. But, in reality, it seems to have affected everything on channel 2 changing the colour a bit.
    Just miss a real presence control and maybe a variable slope resistor (for fun).
     
    Last edited: Apr 18, 2017
  12. robrob

    robrob Poster Extraordinaire Ad Free Member

    Dec 29, 2012
    United States
    Neither of the volume pots have bright caps which would keep the channel from darkening so much when the volume is rolled down. Try a 250 or 500pF cap from volume pot input to wiper on the volume pot of your choice.
     
  13. HF1600ie

    HF1600ie TDPRI Member

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    Ah. You are right. I was thinking on the gain pots!! Hmmm, ok I will try. 250pf means more treble, right ? I mean the more bass will roll-off when the pot is rolled back.
     
    Last edited: Apr 19, 2017 at 4:50 AM
  14. HF1600ie

    HF1600ie TDPRI Member

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    On the first picture, the volume pot of channel 2 is the one further to the right. On the second picture, is the first further to the left.

    In this case, all 3 lugs of the pot are connected to something. The capacitor should be placed between lug 2 and 3 or 2 and 1. Lug 2 is easy, but I don´t know which one is the input.

    Maybe (it´s a guess), I should place the capacitor between lug 2 and that lug in inside the dark green "square" on picture one.

    upload_2017-4-19_11-11-38.png

    upload_2017-4-19_11-12-1.png
     
  15. robrob

    robrob Poster Extraordinaire Ad Free Member

    Dec 29, 2012
    United States
    The dark square should be the ground plane so you'd connect the other two pot terminals with the cap.

    A 500pF bright cap will sound "brighter" because more mid highs will bypass the volume pot. The bass shouldn't roll back with the volume the way the high freqs do.
     
  16. HF1600ie

    HF1600ie TDPRI Member

    Age:
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    Hmm, are you sure it´s ground? The other picture shows :
    The 3rd lug is going into a capacitor (C7 - the presence)
    The 1st lug is connecting to R63.
    The 2nd lug is going a long travel into TCN9 (a connector).

    I thought it would work like a guitar treble bleed. There´s some bass roll-off when you roll back the volume pot, so the treble frequencies will stay in the path. I have to try 250pf and 500pf and see which one I like the most.

    But I am unsure regarding the lugs...
     
  17. robrob

    robrob Poster Extraordinaire Ad Free Member

    Dec 29, 2012
    United States
    In the lower picture, the right lug is the input for the volume pots.
     
  18. HF1600ie

    HF1600ie TDPRI Member

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    OK!

    Thanks. I will let you know the result ;)
     
    Last edited: Apr 20, 2017 at 12:42 PM
  19. Silverface

    Silverface Friend of Leo's Ad Free Member

    Age:
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    Mar 2, 2003
    Lawndale CA
    FWIW whether a presence control or one of the tone controls they are (in these circuits) passive controls that cut frequencies. They don't "add" anything.
     
    HF1600ie likes this.
  20. HF1600ie

    HF1600ie TDPRI Member

    Age:
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    I thought that the Marshall typical presence control would add a "brilliance" on top of the normal tone.
    When the control is turned ON, it would adding some special high-frequencies.
    When the control was rolled back, then, only the normal tone would be heard.

    And in my Vox AC50CPH, the presence is only cutting highs, to the point that when my presence knob is rolled back, the whole tone is muffled which is different from my Marshall, as the Marshall is perfectly normal and trebly, even with presence knob fully rolled back.
     
  21. robrob

    robrob Poster Extraordinaire Ad Free Member

    Dec 29, 2012
    United States
    Negative feedback reduces the gain of all frequencies. A "real" presence control removes high frequencies from the NFB loop so their gain reduction is reduced for a perceived gain boost. The presence control is kind of a semi-active tone control.
     
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