treble bleed with 1meg pot?

Discussion in 'Luthier's Guitar & Bass Technical Discussion' started by rpavich, Jan 4, 2006.

  1. rpavich

    rpavich Member

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    has anyone done this?
    I usually use 1meg pots in my guitars with a treble bleed circuit.
    I finally opened up my Ghia all the way at practice last night, using this setup in my 335 and it retained "too much" highs....just enough to not be "right"...it seemed to work good when I was having to use the hotplate a lot but at full blast it was too much.

    Has anyone used this combo? 1meg pot with a treble bleed?

    Also, would it be fair to say that the treble bleed cap values that people use would have to be "halved" to use with a 1meg vol pot? Considering that most people use a 500k pot?

    bob
     
  2. John Phillips

    John Phillips Member

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    There are two problems with higher pot resistances... firstly that the value of the treble pass cap becomes more critical (ie the difference between say a 220pF and a 470pF cap is more noticeable with a 1Meg pot than a 250K), and secondly that the whole thing becomes more sensitive to the precise capacitance of the cable, which is what causes the treble roll-off in the first place.

    You have to pick the cap value to match the pickups, the pot, the cable and to some extent the tone of the amp too... there is no one right answer. If having got it right you change to a shorter or higher-quality cable, or a brighter amp, you may find that your previous carefully-chosen cap value is now too big and you get that shrillness at lower volumes.

    To correct that you either need to use a smaller cap value, or add a resistance either in series or parallel with it (different values are needed for each scheme) to moderate the effect either by letting less treble through or a bit more bass and midrange... confused yet? ;)
     
  3. rpavich

    rpavich Member

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    John,
    No...not "too" confusing...I figured I'd have to wire some test leads out of my F holes and just try some different values...

    So it's not fair to say this?: 500k + .047 same as 1meg + .002?
     
  4. John Phillips

    John Phillips Member

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    Not as simple as that. In theory, the cap value would be the same for any value of pot assuming you were using the same cable and amp, because what you're doing is balancing the capacitance in the lower part of the potential divider (that of the cable and to a small extent of the amp's input) against that of the upper part (the treble pass cap). But in practice changing the value of the pot changes the frequency balance between the pickup and the pot/cap/cable network, so the result is not constant.

    Even using the same pot value but different pickups does this - my humbucker PRSs have a 680pF cap, which to my ear gives an almost perfectly smooth taper with a 20-foot Horizon cable and a Mesa amp... but my P90 one needs a 330pF cap to give the same result (both with 500K pots). This is because the inductance of a P90 is lower than that of a humbucker.

    But given that increasing the pot value makes the pickups seem brighter, I would guess that using half the value at 1Meg that you did at 500K would be quite a good place to start as you thought, even if it doesn't turn out to be the final value.

    There is also a different wiring scheme you can use, using the tone control to control the amount of treble pass - it's the same as was commonly fitted to single-tone-control amps, eg the Marshall 20-watt. In this, the top tag of the volume pot is connected to the middle tag of the tone control, the low tag of the tone control to ground via the tone cap, and the top tag of the tone control to the middle tag of the volume pot via the treble-pass cap. This allows you to finely adjust the brightness at lower volumes with the upper range of the tone control, and it still works as a normal treble roll-off in the lower range. This might let you dial it in more accurately, and let you compensate for setting the amp differently too.
     
  5. sundaypunch

    sundaypunch Member

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    My experience is that it's difficult to find a treble bleed setup that works across the board. I have a setup I like with certain amps but it is terrible with others. I have amps that clean up really well with the volume knob and others that don't so the need for the treble bleed varies. It is an ongoing battle.
     
  6. rpavich

    rpavich Member

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    Well, here is where I'm at with this...I'm going to try playing without a treble bleed. I notice that it's slightly muddy when turning down but I'm going to give it a shot for a week and see if I can live with it.

    bob
     

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