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    Modified TBX Tone Control




    Fender's TBX tone pot is almost perfectly configured to provide treble cut one way, bass cut the other way, and a no load center position - all with one knob. Unfortunately, Fender doesn't wire it to do those things.


    Here's a schematic of how the standard Fender TBX tone control is wired:

    Fender TBX Schematic

    Here's how it works. There are 2 stacked pots with a single shaft - like a dual gang pot. One of the pots is a 250K Audio taper pot that goes no load at the center of rotation and stays no load from the center to fully clockwise. The other pot is closest to a 1 Meg Audio or Logarithmic taper pot. In the handful of TBX pots I've measured, the 1 Meg log pot isn't quite a normal logarithmic taper. At the center of its rotation, it measures about 1K Ohms (a true 1 Meg log pot would read about 10K at the center position). 

    From Fully Counter-Clockwise to the center, it acts like a typical 250K tone pot, but with an 82K resistor to ground also loading the guitar signal.

    At the Center Detent, the 250K pot goes open (no load) and the 1 Meg pot is only about 1K, so the result is essentially the same as having an 82K resistor loading the guitar signal (in parallel to ground).

    From the Center Detent to Fully Clockwise, the 1 Meg pot starts to increase in value - adding to the value of the 82K resistor loading the pickups. At Fully Clockwise, the load on the pickups is 1 Meg Ohms plus 82K Ohms. That is enough resistance to closely approximate a "no load" condition.

    So, the Fender TBX gets you close to a no load pot, but it doesn't cut bass frequencies.






    So, I was thinking about what it would take to have a single control on a guitar that could do both treble cut and bass cut. Having a no load center position would be a bonus, if possible.

    As I drew it up, it seemed like what I needed was very similar to the Fender TBX tone control. It turns out that the Fender TBX is almost perfect for the job! The only thing that would make it better is if the 1 Meg Log pot were a 0 Ohm conductive strip from Fully Counter-Clockwise to the Center Detent, but its ~1K value at the center is acceptable (I have started using silver conductive paint to mod TBX tone pots now to get my "ideal" setup).


    Here's the schematic of my modified TBX setup:

    Modified TBX Tone Control

    The modified TBX control still has the 250K no load pot attached to the output of the pickup selector switch, but then passes the guitar signal through the 1 Meg pot (with a .001uF cap across it) before it goes on to the volume pot.

    When the value of the 1 Meg pot is low from Fully Counter-Clockwise to the Center Detent, the .001uF cap is essentially shorted out & the 250K pot with its .022uF cap function like a typical tone pot.

    At the Center Detent, the 1 Meg pot is still around 1K (or less) - still shorting out the .001uF cap - and the 250K pot has gone out of the circuit.

    From the Center Detent to Fully Clockwise, the 1 Meg pot starts to increase in value, making the cap the path of least resistance for higher frequencies. The highs will "go around" that resistance and the result is reduced lows. At Fully Clockwise, it's a low-fi type sound.





    And here's the wiring diagram of how it wires up. I've added a 1 Meg resistor across the tone cap to reduce the popping sound that frequently occurs when going to or from the no load center position.


    Click on the image below for a larger version.

    Modified TBX Tone Control Wiring





    Here are a couple of videos I've made about the Fender TBX Tone Control:

    Anatomy of a Fender TBX Tone Pot



    Anatomy of a Fender TBX Tone Pot - Followup



    And here is a video I made showing how I modify the TBX pot itself so that it is better suited for my modified TBX Tone Control.

    Modifying the TBX Pot Itself For Use With My Mod