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Understanding the Fender TBX tone control

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  • Understanding the Fender TBX tone control

    The Fender Telecaster Elite used the TBX tone control, and so a recent discussion suggests understanding what that circuit does.


    Fender claims its passive TBX tone control circuit offers features that you might expect to require an active circuit. Fender is bit vague about what it does, but here is a description from a place where you can buy the modification kit:
    "Some American Standard Strats are equipped with this tone control to cut either treble or bass instead of the standard style that attenuates treble only. When the control is set at the centered detent, it is off. Turning the control one way cuts treble; the other way cuts bass.”


    To summarize the analysis below, a better description of its performance is this: The first thing it does is the first thing a doctor is supposed to do: cause no harm. That is, you do not lose any significant capability, although the response is different with the pot near 0. The second thing it does is to allow somewhat more treble boost than a standard tone circuit, almost as much a tone circuit using a so called “no load” pot where the tone circuit is disconnected at “10”. The third thing it does is to allow good adjustment capability all the way up to the increased treble level. The fourth thing it does is to provide a different taper than you can get with a simple tone control. You might or might not like it; that is subjective and entirely up to the individual.


    It does not cut bass, and it is not “off” at the centered detent.


    To understand the TBX, it is first necessary to understand the standard tone control circuit. A review follows.


    The first attachment shows a simplified guitar circuit. The tone control is the pot and capacitor, but it operates using the pickup coil inductance and all parallel capacitance, including the coil and cable capacitances and stray capacitance. With the tone pot disconnected, the circuit has a resonance, damped so it is broad (low Q). So connect the series pot and cap with the pot on 10, and you damp this resonance a bit. Turn down the pot and you damp it more. Turn it down more and the capacitor becomes important so that with the pot on zero you have a resonance at a lower frequency.

    Click image for larger version

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    The second attachment is a set of frequency response measurements by Antigua from the guitarnuts2 pickup measurement forum. You see just what I described above.

    Click image for larger version

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    Does the tone capacitor affect the higher frequency resonance at all? Well, maybe just a very small amount. The magnitude of the impedance of the capacitor at 3KHz is about 2.4K ohms. This is very small compared to the the resistance of the pot, 250 Kohms, and so the capacitor acts almost like a short circuit until the pot is turned almost all the way down. And then the effect is to make a lower frequency resonance as the attachment shows. So the circuit has a different resonant frequency at each end of the pot rotation with variable damping in between. The next post describes the TBX circuit.

  • #2
    The TBX tone control looks like the first attachment of this post, which is from https://www.premierguitar.com/articl...Control_Part_1. It uses a special dual pot and an extra resistor. I think that the description of what the TBX is is OK (but how the circuit operates is not). For example, if you look at this video https://www.youtube.com/watch?v=EeGaAs1upxM of installation instructions, you get what is described in the Premier Guitar article.
    Click image for larger version

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    The second attachment of this post is a schematic of the circuit, with a description of how the special pot sections work. (The Premier Guitar article has a similar description of the potentiometer sections from which this one was derived.) Let’s start on pot position 0. The B section is a normal tone control and it is connected to the pickup, but an additional 82K is in parallel to the capacitor. (Section A is just s short.) This damps the resonance significantly, and so the response is not quite like shown on Antigua’s plot for 0 since the resonant peak is not quite so high.

    Click image for larger version

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    Now turn the pot to just short of 5, just before the detent. Section B is a regular tone control on 10, but the 82K resistor is in parallel with the C, and so the damping is similar to one of the intermediate curves on Antigua’s plot, not the plot for 10.


    Now move into the detent position. Section B is disconnected, and so the damping decreases slightly. Now turn the pot towards 10. The damping decreases and goes through the “10” curve on Antigua’s plot and almost to the disconnected position.


    A good question to ask is “how is this different from just replacing the standard 250K pot in a normal tone circuit with a 1 Meg pot?” Well, not much. The TBX circuit has somewhat more damping in the 0 position, and the taper is probably significantly different because of the effect of the 82K resistor..

    Comment


    • #3
      Yeah I'm sure it would be a lot simpler to create a 1 meg tone pot with some sort of center detent than to have a custom dual gang pot. I like the TBX control for the fact that it sounds "normal" at the center position, and increases the Q in one direction and lowers it in the other. At $12 the price isn't bad.

      Comment


      • #4
        I suppose they could have made control that cuts bass effectively as well as treble in the other direction, low-loss and with a centre off detent. Similar construction with a tandem pot, but with one section (0 to 5) A250K for the treble cut and the other section 5 to 10) C1M for the bass cut. Wired up the same as the G&L bass/treble tone circuit. This would run fairly smoothly and give a wide tonal variation.

        Comment

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