"A lot of people think of the TBX tone control as a treble boost, but that’s not quite accurate. The TBX control actually consists of a custom dual-ganged pot (aka a “stacked” or “stereo” pot), a resistor, and a capacitor that cuts the bass and treble out of the circuit, depending on which way you turn the knob. This can add some new dimension to your solo parts, especially if you are going for those bright, crystal clear Jeff Beck tones.
The basic configuration of the TBX control (Fender part no. 0992052000) changed several times over the years. Fender used several different values for the two pots, the capacitor, and the resistor. The first few versions also lacked a center detent function. The current version consists of a detented 250k/1 Meg stacked pot, a 0.022uF standard film capacitor, and an 82k-ohm carbon-film resistor. In a nutshell, the TBX tone control is a special pot that cuts either treble or bass instead of a normal tone pot, which cuts only treble. This is done with the dual-ganged pot, which is wired to work as a low-pass filter in one direction and a high-pass filter in the other. The center detent in the middle is provided for the off or “flat” position.
The dual-ganged pot is cleverly designed, meaning you can’t substitute a normal stereo pot to make your own budget TBX control. How does it work? The bottom pot (with the shaft up) is pot B on our drawing and is the normal tone control we all know. It’s a standard 250k audio pot with a range from 0 to 5 on the knob. At the detent (middle) position, it goes open and acts like a no-load tone pot, remaining out of the circuit from 5 to 10 on the knob.
The engineering behind this is actually very clever. Normally, the resistive material ring inside of the pot is a band of carbon-containing gunk that is printed onto the phenolic wafer. On the lower TBX pot, only half of the ring is conductive, as the other 50 percent is made out of a non-conductive material. So we can say it is a no-load tone control pot, but instead of going open at approximately 98 percent of its rotation, it goes out of the circuit at exactly 50 percent.
The other pot, which is labeled A, acts in the opposite direction. It also has a split resistive material ring inside, but instead of non-conductive material, metal is used for one half of the ring. This means that between 0 and 5 on the knob, its resistance is at maximum. After the detent position, the normal function takes place from 5 to 10 on the knob. This 1 Meg linear pot comes into the circuit in series with the resistor after the detent position. Because of the high resistance (1 Meg ohm), the load added to the passive guitar circuit is very low"
There is a zillion ways to build these to fit most any application