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How To Balance Pot Tapers/Treble Bleed

Discussion in 'Luthier's Guitar & Bass Technical Discussion' started by Ayan, Feb 23, 2005.

  1. Ayan

    Ayan Member

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    I just posted this at the LP Forum, and thought some here may also enjoy the read. Hope so. :)

    - - -

    I have noticed that some folks here have posted describing how the volume controls on their guitars have different a "feel" after they've upgraded the pots. More often than not, these have been upgrades of 300K linear pots with 500K CTS audio pots. Along with the upgrade, many opt to install a "brightness network" on their neck pickup volume pot, and that's when they notice the difference between the way in which the two volume controls respond.

    I have seen replies to these posts wherein it is explained that the resulting different taper on the neck pickup pot is normal. Of course I agree with that statement, but what I find curious is that no additional information is provided to make further changes that will equalize the taper of the two pots.

    For reference, I have been putting "brightness" mods on my guitars and volume pedals for the last 20 years or so, as I never liked treble roll off effect that results when turning a volume control down. There are numerous ways of going about this, and many pickup manufacturers offer suggestions in their websites --and I know most of you are hip to them. However, in guitars with more than one volume control, one may want different "treble retention" responses from each pot, and that is something I quite honestly haven't seen posted anywhere. That being the case, let me offer the following examples as mere guidelines to you. Above all, I suggest you experiment until you get the sound you want, since no two guitars/amps/pickups/tastes/ears/etc. are the same.

    Without any further adieu, here is what I use in all of my gutars:

    1. Single coil guitars equipped with a single 250K audio pot: I use a 100K resistor in parallel with a 1,000 pF capacitor between the input and output lugs of the pot. [ Yes, there will be differences depending on whether you select a carbon comp VS metal oxide resistor VS... and silver mica VS ceramic VS polyprop VS mylar VS.... caps, etc. ]

    2. Volume pedals with a 250K audio pot get the same cap and resistor as 1.

    3. Humbucker guitars with two individual volume controls: (a) For the neck pickup, I use a 220K resistor in parallel with a .0047uF cap. This allows for the tone of the neck pickup to lose a little bit of its bottom end when the volume is turned down. It definitely controls the "mud" factor when using overdrive, while preserving the midrange; (b) For the bridge pickup, I use an 82K resistor in parallel with a 1,000pF cap. This basically causes the taper of the bridge pickup pot to "keep up" with the taper of the neck pickup pot, but it doesn't really emphasize the high end as you roll the control down.

    So, if you have your "brightness" network on the neck pickup pot and you like it (I couldn't live without mine), you don't have to live with the taper inbalance between the two volume controls anymore.

    Just a couple of final notes:

    1. If you like the feel of an audio taper pot, where basically the volume control is very slow until you hit 7 or so and then it takes off, perhaps you may want to try the 50s wiring of your tone control (where the tone cap connects to the wiper of the volume control, as opposed to the input lug) instead. Otherwise, adding any brightness network will "speed up" the taper. [ Note - I don't own a vintage guitar, but I believe that old audio pots used in guitars may have had a 30% taper -- this was certainly the case with vintage amplifiers -- which is quite a bit closer to linear than to the 10% taper that most (except custom made) new pots have. ]

    2. If you want to experiment further with different resistor and cap values, I am sure that most of you will already know the rule: a lower resistor value will accelerate the taper more and more, and it will tend to be less and less frequency selective as the resistor value becomes smaller. Increasing the value of the cap, if you start with a very small one, will make the sound brighter and brighter until you reach a certain point. After that, the cap will start passing midrange frequencies as well... And in the extreme case, if you put in a large enough cap it will be equivalent to short circuiting the input and wiper lugs on the volume pot.

    I hope this helps a few of you out there who have expressed the desire to mitigate this issue. The cost for these mods is very small, but they can go a long ways if you unleash the power of riding your volume controls to get different tones on the fly.

    Cheers,
     
  2. Mark Robinson

    Mark Robinson Member

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    Great Post Gil, I've never seen better on this subject. Might just have to pop into my local electronics store on the way home, Thanks!:D
     
  3. KLB

    KLB Member

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    Gil,
    Very informative. Thanks.

    What resistor and cap types do you prefer for guitar tone/volume use?

    "I'll have a 1/4 watt Carbon Comp with Polystyrene bypass, please mamm"

    Cheers,
    Ken
     
  4. Ayan

    Ayan Member

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    Ken, all of the 100Ks I use (on a Strat, a Tele and 2 volume pedals) happen to be carbon comps (Allen Bradleys that I had lying around) that I paired with some high quality ceramic discs. My humbucker guitars have carbon film resistors and Xicon MPP caps.

    Gil
     
  5. iwo

    iwo Member

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    Ayan, could you give me some advice... what would you change in your given suggestions for humbucker guitar if you would have a neckpup volume 500 kohms but bridgepup 250 kohms? I guess the resistor should be larger cause the resistance pup sees would be too small and the high frequencies would start decreasing cause of this?
     
  6. skylabfilmpop

    skylabfilmpop Silver Supporting Member

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    I was told by a friend who makes our "favorite amplifiers from southern California" that resistor values in a treble bleed (and in general) work inversely to their size. That is a 220k will allow a less pronounced amount of hi end to pass and an 82k will allow more. Toward that end I like a 150 -220k resistor with a 1000pf cap on the bridge depending on the character of the pickup (less for a dark one and more for a bright one) for the bridge and usually a small 82k or less resistor with a 1000pf cap on the neck. What this gets me is very neutral effect on the bridge pickup, the tone remains the same as you turn down and on the neck pickup a treble boost as you turn down yielding very stratty neck tones. I didn't hear much difference between the 1000pf cap alone on the neck (ala 50's teles volume knob bright cap) or with the 82k resistor but the resistor prevents the cap from dialing out half way through the pots range. the larger cap on the neck is a good Idea (I have been wanting some David Hidalgo goldtop neck tone myself) I will have to try it out.
    Regarding the other posters consideration with a humbucker with a 250k pot. If you are using that value to combat an overly bright pickup, try a higher value resistor (eg 220 k) with a 1000pf cap and see how that works for you. The Treble bleed circuit seems to my ears at least to reintroduce some high end normally sucked off by the tone cap being connected to the pots input, even on 10, so a larger resistor will also serve to minimalize this effect. If you don't hear enough hi end as you turn down lower the resistor value and if you want more mids to pass along with the high end raise the cap value.
    For those curious about results with 50's wiring I have reluctantly come around to the RS superpot. Initially I thought the pots were hype. I don't really think they sound any different than a standard CTS pot of the same value BUT the taper is perfect for a volume pot in a 50's style wiring schematic, and avoids the abruptness caused buy wiring the tone cap to the pots output. Currently I am in the process of replacing my bridge volumes in all my Gibsons with an RS superpot and NOS caps.
     
  7. Tone_Terrific

    Tone_Terrific Member

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    Resistors can go in series with the cap to control the amount of bass rolloff and not mess so much with the taper.
    Clip your connections off and jumper to experiment.
    There are situations, IMO, in which topend loss is a good thing and vice-versa, but adding switches and more trimmer pots for choice is overkill when it comes to stage usage.
     

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