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Discussion in 'Pickups & Electronics [BG]' started by Fong249, Feb 3, 2006.

  1. Fong249


    May 25, 2005
    Washington, DC
    I'm about to rewire my Jazz bass, and i've worked with pots before, only on guitar. What exactly are caps for? How do i know which one is for me?
  2. The caps are part of the tone control circuit. In simple terms you can think of a cap as a resistor that decreases with frequency, i.e. it has high resistance at low frequencies and low resistance at high frequencies.

    The tone control is simply a pot that connects to ground through a cap, short-circuiting part of the signal (how much depends on the position of the pot). Since the cap has low resistance at high frequencies, it short-circuits them more than the lows, hence the treble cut effect of a passive tone control.

    Having said that, if you take into account the whole circuit, including the pickups with their own inductance, resistance and capacitance, the volume control, the cable capacitance, etc., the effect of the cap turns out to be significant only at low values of the tone pot (say below 5 or so), and with the tone pot fully open, its effect is negligible.

    There are two variables you can play with when selecting the cap: its value and its type/brand, the first one being the one that has a more dramatic effect when you change it. Normal values used in basses and guitars are 0.01, 0.022, 0.033, 0.047 and 0.1 uF. The bigger the cap, the more noticeable its effect when you turn the tone down. AFAIK the standard value for a vintage Jazz bass is 0.047, but if you want a subtler/stronger effect, you can try smaller/larger values.

    As for the type/brand, rather than opening that can of worms, I'll just refer you to this recent discussion on the subject in The Gear Page (jump ahead to the second page which is where the fun started): http://www.thegearpage.net/board/showthread.php?t=125313

    Now, as mentioned in that discussion, one thing that does affect the tone significantly even with the tone control fully open, is the value of the pots (and this goes both for the volume and tone pots). The standard pot values used in most guitars and basses are 250k and 500k (some also use 1M pots and in active circuits you will find much smaller ones, down to 10k, but here we're talking passive circuits). Every pickup has a resonance (a peak in its response) at a certain frequency that gives it its characteristic "voice", depending on the frequency and on the height of the peak. What changing the pot values does is adjust the height of the peak: 250k pots dampen the peak more than 500k pots, so in general you get a warmer sound with the former and a treblier one with the latter. For intermediate results you can use for example a 250k volume pot and a 500k tone pot.

    In basses, including a vintage Jazz, you normally find 250k pots, but you can replace some or all of them with 500k pots if you want a brighter sound.
  3. Fong249


    May 25, 2005
    Washington, DC
    thank you very much. I'm going for the vintage tone, and already ordered two 500k pots for volume and 1 250k pot for tone. i think i'll go with a .047 cap then.