Adding a "Bright" cap

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by ComingApart, Jun 29, 2006.


  1. ComingApart

    ComingApart Member

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    How would one go about adding a "Bright" cap to an amplifer thats a bit dark in tone.
    It's a master volume amp, so would you solder the cap to the master volume pot or the pre amp pot?
    Also I read it mentioned that soldering a resistor in parallel with the cap would help the treble through the range of the pot, as opposed to the first bit of travel. Could someone explain the theory behind that?
    Also, what would be as good way to determine ballpark figures for the cap and/or resistor?
    Thanks.
     
  2. hasserl

    hasserl Member

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    A typical Bright cap is a small value cap placed between the input terminal and the wiper terminal of the volume pot, not the Master Volume, the regular Volume control. It might be labeled Gain or Drive in some amps. The larger the cap the brighter the effect. I tend to favor about 100 to 150pf, but other values have been used in different amps, 250, 500, up to 1000pf in some Marshalls. Those tend to be overly brght and are the ones people like to clip out.

    If your amp is a pcb amp with the pots mounted to the pcb adding this cap is difficult. If the pots are mounted to the chassis and connected to the board by flying leads it's a piece of cake. Identify the input terminal and place the cap between it and the center terminal. Hint, the input terminal will be one of the outer two terminals, the one that isn't the input will be connected to ground.
     
  3. doctorx

    doctorx Member

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    Good accurate post by Hasserl. I just refurbished a Kay 720, which was a real dark bass amp, into a great guitar amp.

    One of the things I did was put a variable bright switch in it. I used a three way rotary switch, selecting either a 47pf, a 120pf, or nothing.

    It gives me a choice of brightness for when I use my Les Paul, which needs a little brighter tone, or my Strat, which needs a little less.

    This amp had three inputs, I put the switch in one of the jack holes.
     
  4. Swarty

    Swarty Member

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    To address the OPs second question... If you have the cap on the pot, as long as the pot is presenting resistance to the signal a portion of the signal (upper frequencies as determined by the cap value) will pass through the cap. As the pot is turned up there is less resistance, thus more of the signal flows through the pot and less through the cap...until you have the pot turned all the way up and all frequencies pass. If you use a fixed resistor w/ a cap there is no variation in the equation. On Marshall 2 holers there is a cap w/ resistor at the input to the preamp volume control. On a Marshall 4 holer there is a cap on the mix resistor of the lead channel. Seems like for both amps the the values are 470k and 470pF (early 4 holers would have a 270k).
     
  5. hasserl

    hasserl Member

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    The formula to determine the frequncies effected by the high pass filter Swarty described is:

    Formula F = 1/(2*p*r*c)

    Where F = Hz, R = Ohms and C = Farads

    I have an Excel worksheet that will do the computing for you if you input the resistance and capacitance. Anyone want a copy shoot me a message and I'll email it to you. (you do need Excel installed for it to function).
     
  6. donnyjaguar

    donnyjaguar Member

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    Here's a thought. The bright cap does less and less as you turn up the volume. If you're running full-bore its actually doing nothing at all. I added a "Presence" cap to one of my amplifiers that was a little dark sounding and now its bright at all volume settings.
     
  7. ComingApart

    ComingApart Member

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    By that I'm assuming you mean you installed a cap on your Presence pot?
     
  8. donnyjaguar

    donnyjaguar Member

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    No, its switchable - just like a brightness switch. You have to play around with capacitor values until you find one that's just right. In my case it was 56nF. I guess I could've put a presence control in there, but didn't really find the need.
     

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