Adding a contour knob to an amp?

Discussion in 'The Squawk Box' started by Goodcleanfun, Oct 16, 2011.

  1. Goodcleanfun

    Goodcleanfun Senior Member

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    I've been wondering about this for a while. I like my amp (modified Crate V3112) a lot, I just find myself wondering about adding a contour knob. My Valvestate had one, the Marshall Shredmaster pedal had one, the Jekyll/Hyde pedal has an equivalent and now I'm curious as to how one could be implemented into the circuit I have.

    One part of me is thinking I could rig up a small PCB, basically the important parts of a pedal design and implement that into the PCB, passively powered and when at zero it's bypassed... but this sounds like a lot of work, but there's enough space in the amp for this to be done I suspect.

    Second choice is to use what's in the circuitry already present on the PCB and just rig a knob up to a component, or between some components to bypass a certain part of the gubbins... I just don't know what the gubbins are.

    Here is a schematic:

    http://www.blueguitar.org/new/schem/misc_amp/crate_v32_palomino.pdf

    I think I established that R80 controls the 'presence' function of the amp, which I'll be wiring to a pot instead of just the all-or-nothing switch that currently is on there, but if anyone has any idea about the addition of a contour knob for a scoopier/smoother sound please let me know. I'm all ears.
     
  2. Lyle Caldwell

    Lyle Caldwell Senior Member

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    Variable "presence": replace R80 with a 100K linear taper pot using lugs 2 and 3 of the pot, followed by a 47K resistor to ground.

    Variable "contour": Lift one end of R13 and put a 25K linear taper pot between the free end of R13 and the board connection you lifted R13 from. Doesn't matter which side. Be aware that this new pot will have 195VDC across it, so use a 1/2W pot and make sure you use an insulated knob.
     
  3. Goodcleanfun

    Goodcleanfun Senior Member

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    Lyle, once again I owe you thanks.

    If you were here i'd kiss you. On the face.
     
  4. Goodcleanfun

    Goodcleanfun Senior Member

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    One of these for the contour pot?

    About the knob... is it safe without an non-insulated knob? Am I at risk with a regular pot? I'll be getting an insulated one but I'm curious.
     
  5. Lyle Caldwell

    Lyle Caldwell Senior Member

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    OK, add Ithaca to the "cities to avoid list". :D

    So long as you don't touch the lugs or connections on the pot you should be completely safe. Once installed in the chassis, the pot casing will be at ground, so between your fingers and the 195VDC will be ground, then the plastic knob.

    Wasn't trying to scare you, I just wanted you to know about the high DC on that pot.

    Any standard 1/2W pot is fine (Alpha 24mm, CTS, CTE).

    One of whats for the contour pot?
     
  6. Goodcleanfun

    Goodcleanfun Senior Member

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    there should have been a link there to this:

    [ame]http://www.amazon.com/Parts-Express-25K-Ohm-Potentiometer/dp/B0002KRE2A[/ame]
     
  7. Lyle Caldwell

    Lyle Caldwell Senior Member

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    That'll work fine.
     
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