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A novel kind of guitar Tone control

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  • A novel kind of guitar Tone control

    In the process of responding to a question about compensated Volume pots the other day, I stumbled into an idea which intrigued me, so I tested it out last night, and am pleased to report it works as predicted, and delightfully so.

    As readers are aware, for a variety of reasons there tends to be brightness lost in the guitar signal as the volume pot is turned down. The traditional solution for this is to use a "bypass capacitor" that straddles the input and output of the volume pot such that whatever attenuation is applied to the overall signal is not equally applied to the top end of it. There are a variety of bypass schemes which I won't debate the relative merits of, but they all follow the same general plan.

    If one examines the Tone/Volume controls on any of the early Fender amps, one sees an interestig configuration in which the Tone control has a reciprocal action. The wiper of the Tone pot is connected to the input of the Volume pot, and has caps on each end of it that do different things. You can see an example of it here:

    In one direction the tone pot bleeds treble to ground, as usual. In the other direction, it provides a bypass cap across the volume pot, but not until the resistance on that leg of the Tone pot is low enough. Of course, since the Tone control has a reciprocal action, it does not reach that minimum sufficient resistance until such time as just about all possible treble bleed out the other side of the pot is minimized.

    What occurred to me is that one could use this to provide two different degrees of treble bypass on the Volume pot. The Volume pot would have its default bypass arrangement, using a smaller value cap, and when the Tone pot was maxed, a second cap would be added in parallel, providing treble bypass at a lower corner frequency.

    Why do this? On my guitars, I like to use an over-valued bypass cap, like 1000-1500pf, such that turning down the volume to around 7 gets me more of a bass cut than an overall volume cut. I like this for making humbuckers or P90s sound a little more single-coilish. Basically, I like it if I want to do chicken-pickin or Motown chicka-chicka scratchy rhythm. As a bass-cut it works better than coil-cancelling because you still retain the hum-cancellation. However, once installed, you can't really defeat it easily.

    Enter the variable-compensation tone control (I'll copyright it as VCTC ).

    Last night I tried it out. My volume pot is 500k, with a 470pf bypass (that's the way, uh-huh, uh-huh, I like it). The tone pot is 1M, though this might work better with 500k. The treble-bleed cap on the tone pot is .022uf. The cap that goes between the other outside lug of the tone pot and the wiper of the volume pot is 1000pf. So, when the tone pot is turned down a bit from max treble, there is a substantial resistance in series with the 1000pf cap and the effective bypass in the volume pot is only the 470pf. If I turn the tone up to max, that resistance is removed, and the bypass capacitance now becomes 470pf+1000pf = 1470pf.

    Voila! I get to have my cake and eat it too. The additional cap has no impact on the treble bleed functioning, and of course since it is a bypass, it has no impact on tone when the volume pot is maxed. One cool aspect is that when I turned my Tone down to about 6, and turned the volume down to around 4, I could increase the "snarl" in the already-clear guitar signal by simply turning the tone pot up.

    The values are not written in stone. The basic principle is simply to let the tone pot add more capacitance to the bypass path and let more upper mids through at lower volumes. Give it a try. You might like it.

  • #2
    That's an interesting idea. I'm going to have to play with this some.

    That chicka-chicka bypass cap tone was used a lot on Teles. You can hear it on stuff like Sly and the Family Stone. I always liked real thin sounding guitars for funk.
    It would be possible to describe everything scientifically, but it would make no sense; it would be without meaning, as if you described a Beethoven symphony as a variation of wave pressure. Albert Einstein


    • #3
      So, to test this, all I would have to do is add a cap from the wiper of the volume pot to the "other end" of the tone pot?
      ST in Phoenix


      • #4
        Yes. Pretty darn simple, isn't it?


        • #5
          Awesome and brilliant!

          Do you use any resistor with your normal bypass cap? I use a combo of resistor and cap in parallel for my vol pot bypass, sometimes a second resistor in series with the R||C pair to tweak the taper. Both are to keep the tone from getting too thin at lower settings.

          FWIW I have seen the term "treble bleed" applied to the cap used between the vol pot hot & wiper, and yet I see it used in the tone control description above to describe a shunt to ground. Can we live with this kind of confusion?


          • #6
            Personally don't use a series resistor for the default compensation cap, but have no objections to its normal use in other people's guitars. A matter of personal taste, rather than appropriateness.

            More importantly, though, there is a need to use a fixed resistor in parallel with the tone pot leg that is placed in series with the added bypass cap. One would like for there to be something going on with respect to audible tonal changes between the "2" and "9" positions on the tone pot. I'm going to suggest that one stick a 100k-150k fixed resistor between the wiper of the tone pot and outside lug connected to the extra bypass cap. That way, the added mids provided by the bypass cap can kick in without having to rotate the tone pot all the way.


            • #7
              IIRC I started using RC combos in my guitar vol bright circuit because a small cap alone had too high of a corner frequency for my taste, but using a bigger cap alone made the effect too pronounced. I also like the way the R smooths out the taper.

              I have been thinking about using a push-pull vol pot to switch the bright circuit in and out, but your idea might be a nice alternative.


              • #8
                I guess you are aware of this:

                Fender Products: TBX Tone Control

                I had that on a couple of Fenders and it does what it says it does. The down side to it is that it is somewhat abrupt under "5" and way to bright above "8", so I have take it out and replaced with a conventional tone control. They had another scheme that actually disabled the tone control completely at "10", but I've never tried it.

                I got to give your idea a shot though. Thanks for coming up with that.



                • #9
                  Leo thought of a lot of different tone circuits:

                  TONE CONTROL FOR STRINGED INSTRUMENTS - Google Patent Search
                  ...and the Devil said: "...yes, but it's a DRY heat!"


                  • #10
                    Extremely efficient frequency compensated guitar volume control.
                    Attached Files
                    It's All Over Now


                    • #11
                      I tend to agree with Groover. The cap alone, even if smaller than usual, doesn't track well to my ears. I tend to use a resistor roughly the value of what the effect pot resistance is more or less.


                      • #12
                        Schematic updated

                        But that "compensated" idea looks like a tone killer. I quite agree with adding that smoothing-resistor in series to the treble bleed cap, sounds better to me too. The nice thing about this is that, the guitar just "works" - like a Mac. You don't need confusing switches. But I better try this, err, first...
                        Is this schematic right enough? [edit: now it is.]
                        Attached Files
                        Last edited by Guitarist; 07-01-2010, 08:40 PM. Reason: added the Tone-taper resistor


                        • #13
                          Interesting, though it looks like it is essentially a midscoop network. The use of 220k resistors seems a bit odd, though, and demanding of a very high-value volume pot in order to do anything noticeable.


                          • #14
                            That's 99% correct. Stick a 100k-150k fixed resistor between the wiper of the tone pot and 1000pf cap, and you're in business. That will provide a better taper for nudging the mids more with less pot rotation, without deleteriously affecting the treble cut function.


                            • #15
                              About frequency compensated guitar volume control.
                              The basic idea came from the Standel FB-5 equalizer. Schematics is a bridged T filter for approximately 500Hz. Volume and tone pots are standard guitar values (250 - 500kOhm).
                              At lower values of volume pot, and tone pot in boost (right) position, is no too much effect treble boost capacitor 470 pf, so that the tone is too sharp, but it is pleasant to the ear. T filter (2 x 220kOhm/ .0047uF) is a middle cut to approximately 500 Hz, whose effect gradually disappears at higher values of volume pot. In middle position tone control, guitar tone is flat, that would gradually go to treble cut (tone pot in left position).
                              By reducing the volume control, does not change the character of the guitar tone. Relationship of high and low tones are independent of the position of the volume pot. In other words volume pot is frequently compensated for the desired frequency (in this case it is 500 Hz). Each guitarist choose the frequency that suits him, for his ear, and calculated values for the bridged T filter and treble cut capacitor (.01-.047uF)

                              Schematics is extremely simple. Try it, and after we can talk.
                              Last edited by vintagekiki; 07-02-2010, 04:50 AM.
                              It's All Over Now