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What's this extra cap for? Ibanez musican content

Discussion in 'Pickups & Electronics [BG]' started by ampegfuzz, Oct 27, 2013.

  1. ampegfuzz

    ampegfuzz Supporting Member

    Feb 13, 2007
    Davenport, IA
    The volume knob on my '79 musician bit the dust at the gig Friday so I have to replace it. No prob till I noticed something. This extra cap:


    Between the first and second lug is a .331 cap. I know it's not the tone cap. Is it there for comb filtering or to retardd the output to the preamp (which is gone and now passive). Ok to remove it when I replace the pot? I'm still in the air on a preamp so probably going to keep it passive since the pre was Mia when I bought the bass.
  2. bassfreakah


    Mar 26, 2011
    Endorsing Artist Ernie ball strings
    My passive peavey foundation has this on the volume pots too.
  3. Looks like a 330pf treble bleed arrangement. It's not unusual on a guitar, but sometimes a circuit like this can be useful on an all-passive bass (depending on your preferences/tastes).

  4. It's to prevent loss of high-end when lowering the volume, which happens on all passive basses.

    You can leave it, but obviously you will lose that.
  5. ampegfuzz

    ampegfuzz Supporting Member

    Feb 13, 2007
    Davenport, IA
    Am I loosing any output or low end having it? Volume is either on or off. So keeping high when when rolling down is moot to me. I'm still loosely contemplating a Bart pre or a booster circuit.
  6. No, you would not lose output or low end.
    I do wonder if you have a bad pot since you say, "Volume is either on or off". If the pot is the culprit, I would replace it with the same value pot. I might keep the 330pf in there if that's what it originally came with, or likely wire in some jumpers with clips to try different caps (or no cap at all) to see how it works and sounds.
  7. ampegfuzz

    ampegfuzz Supporting Member

    Feb 13, 2007
    Davenport, IA
    The pot is frozen in full volume. That's why I'm replacing it. I would 100% leave the cap if the preamp was still there. With it gone, just seeing the caps usefulness in its current, always passive, state. Going to try the alligator trick with the cap before I replace the pot. Is there a easy way to tell if it's 250 or 500k pots?
  8. Which model Musician is it?

    You could check the pot value with an ohm meter (DMM or analog) after disconnecting the wires. Or, see if you can read all of the info stamped on the pot case.

    With the pot full up the 330pf is essentially out of the circuit and you won't really hear its effect. It comes into play as the pot is "rolled down", bypassing signal (as a high-pass filter) around the pot.

    Here is a schematic of the MC924 preamp I found in another post:

    It shows a 500k volume pot, and a 300pf bypass cap as stock.
  9. ampegfuzz

    ampegfuzz Supporting Member

    Feb 13, 2007
    Davenport, IA
    It's a 1979 mc900 863E8BE5-6F87-4153-B460-264ED42064FB-616-000000C7BEC3A6D7_zpsb26b57f7. . It's had a ruff life but still plays like a dream.
  10. ampegfuzz

    ampegfuzz Supporting Member

    Feb 13, 2007
    Davenport, IA
    Cool, looks like my pickup switch may be aftermarket too.
  11. bassbenj


    Aug 11, 2009
    I would leave it in. I've installed these on some of my passive SX.

    It is indeed a "treble bleed" Cap and the value is not .333 mfd but rather .000333 mfd. Because changing the volume setting changes the impedance seen by the pickups it also changes the tone (just like a 500k volume pot sounds brighter than a 250k). When the volume is up full you'll get a nice full bright sound, but without the cap even turning down 10 or 20% wiil sound as if you turned up the tone control some. Personally, I never turn down the volume a lot but only 10 or 20% to give me a bit of adjustment range on the bass if needed. But even that much will roll off highs a bit.

    Testing the cap on the broken pot won't work because it's frozen at full on. The cap does nothing at that position. Test it with the new pot.

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