Poor range from tone pot

Discussion in 'Pickups & Electronics [BG]' started by MothBox, Dec 23, 2013.

  1. MothBox


    Oct 25, 2010
    I've installed a home-made veritone cap selector linked up to a 500k tone pot but I'm finding in the lower cap values (0.022, 0.047, 0.068) the variation in tone is incredibly limited using the tone knob. With 0.022 its difficult to hear any difference at all although listening very closely there is a change in tone as the cap is rolled on and off.

    Having used 0.022 caps before (no veritone) I'm certain the response should be greater than this and therefore I'm not sure why.

    Does this effect imply that the circuit is shorting somewhere and therefore grounding alongside the cap?

    The higher values (0.1, 0.15) work fine, although maybe a little small in their tonal range.

    Has anyone experienced this before or got any suggestions for things to check?
  2. The lower the capacitance, the less signal is cut. For example, an extremely high capacitance would kill everything in the audible spectrum, while an extremely low capacitance would only cut frequencies above the human hearing range. It sounds like you are losing a range of control, because changes in resistance don't equate to large perceptible differences, as the capacitance decreases.

    If you want to optimize the usable range of a tone control, first off, try a lower value pot, to "stretch" the usable range of resistance over the greatest area of the sweep of the pot. Second, be sure you have an audio taper pot. Linear tapers don't reduce the resistance to a usable range until quite far in the rotation.
  3. MothBox


    Oct 25, 2010

    So a 250k log pot would give a better response range?

    I've read that 250k pots give a darker tone. I assume that means bassier. Is that bassier response something that can be adjusted by way of the tone pot itself? i.e. the bassier tone is only present when the pot is fully closed and everything from closed to fully open is as you'd expect with a 500k?

    Is there a quick way to check if the pot is linear or log? I can't see anything written on the bottom of the pot.
  4. Since the resistance is only in series with the capacitor, 250k pots are identical to 500k pots from 0 to 250k. However, there is no 250k to 500k setting, so your "10" setting on a 250k pot is like an "8" on an audio taper 500k pot. Get a no-load pot if you want a bypass setting.

    Usually, on stamped pots "A" denotes an audio taper and "B" denotes a linear taper. (For example, A250k or 250kA.) If your pots aren't stamped, you will have to measure.
  5. lundborg


    Apr 8, 2008
    If you set the pot in middle position ("5"), you should have equal resistance to each other terminal, if the pot is linear. E.g 250 and 250 for a 500 k pot. On a log/audio pot it will be more like 50 and 450 (or something like that).
  6. MothBox


    Oct 25, 2010
    Are there any issues related to having a 500k (log) vol and a 250k (log) tone pot? Is it recommended to keep them both of the same value?
  7. Nope.
  8. michaelandrew

    michaelandrew The bass player is always right.

    I've just recently been experimenting with different passive control set-ups on an Squier VM Jazz (see "tricked out Squier" thread). I get best results - for me - using 0.1 uF with a 500K Audio taper tone control, but I've only tried 0.047, 0.1 and 0.2 uF.

    I also tried a 500K audio taper "no load" tone control with the 0.1 cap. These old ears (I'm 58 and hear very little above 14 KHz) cannot hear any difference whatsoever between 500K and "open" (meaning disconnected entirely) in the tone circuit - but YMMV.
  9. 500k is basically removing the capacitor from the circuit, for most pickup impedances. There is no reason for bypass options with 500k pots.
  10. MothBox


    Oct 25, 2010
    So with a 250k id also need a bypass on my veritone (homemade) to get a truely un-interrupted tone?

    This isn't a problem, i can remove a cap from the veritone and if the end result is a better range of tone from each cap with the tone knob I'll head down that route.
  11. MothBox


    Oct 25, 2010
    Ive heard of these no load pots but have only seen them on the Fender website. My local electronics store doesnt seem to know what I mean.

    Ive been using the 0.68 cap more than any other for general fingering. 0.22 works really nicely for chords, i used to switch the balance toward the bridge but with the 0.22 and mid balance ive found a really nice tone. So far really pleased with the changes but just need some tweaking.
  12. It is a pot that clicks off at the end of the rotation. Just like the volume knobs on radios.

    In any case, you can use a push/pull pot if you want.
  13. MothBox


    Oct 25, 2010
    I assume the no-load click off creates a direct bypass? The same as clicking a veritone to a no-cap setting.

    I'll play with the pots in the store and see if any of them have the click off. I'm really anal about the feel and sound of switches and pots so want to spend a bit of time turning them and clicking in the store to find what I want!
  14. Yes.
  15. GlennW


    Sep 6, 2006
    Ime, 250K pots choke the tone by removing presence. I like a bassy bass, but use 500K pots when I use pots. Often I'll use a 500K volume pot and a tone (cap) switch with no pot.

    I tried a 100K tone pot, a complete waste of time.

    Are you sure your caps are uF and not the "right" number value on a different scale? Unlikely, but possible.

    As someone else said, for a simple starting point try a 500K pot with a .1uF cap.
  16. MothBox


    Oct 25, 2010

    I need to take the pot out to check it properly on the top for any additional lettering etc to confirm is a log pot and not linear. Using the veritone with .1uf the tone know works fine, maybe a little limited but there is a noticable variation in tone. Its just the lower caps (0.047, 0.022) that don't have much of a range. The 0.022 is a Jenson bare-knuckle so its a guitar specific cap, no room for mis-buying of caps.

    I reckon Line6s' suggestion of stretching the tone over a 250k pot may make for an improvement.

    The vol pot leads off quickly with about 1/3 of the rotation being almost silent then a steep increase in volume toward the end. I think I may need to review that too and get a linear pot for that instead of, what I assume is a log.
  17. michaelandrew

    michaelandrew The bass player is always right.

    I used a 500K Bourns "No-load" pot - available on-line from Newark Electronics - but, as I and Line6man have said, there's no audible difference so I wouldn't bother.