The Fender TBX Tone Control, Part 1

This month, let’s take a look at the Fender TBX tone control mod. This is one of the oldest factory mods from Fender—so old that no one seems to remember exactly when it was introduced. I tried researching this, but found many different dates, and even the guys at Fender Germany weren’t quite sure. As far as I can remember, it was introduced in 1988 along with the Eric Clapton Signature Strat, but I would encourage anyone with more specific information to get in touch. (I find it interesting that the history of this unique control is growing hazy after only 20 years. No wonder a lot of details about the golden days of guitar building have been lost forever.)

Fender.com describes their TBX tone control—which stands for “Treble Bass Expander”—as follows:

“This detented, stacked 250k/1 Meg control enhances your tonal palette without the use of a battery. From 0 to 5, the TBX is your standard tone control, but once you pass 5 you start to decrease the resistance, which allows more bass, treble, presence and output to flow to your amp.”

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.

The diagram shows you how to wire the TBX tone control on your Strat. The red wire is the input for the TBX control, and the green wire is a short jumper wire, connecting pot A to pot B. The TBX control can be wired as a substitute for any normal tone control for any pickup, as well as a master tone control for all pickups. Any mod that works with a normal tone control works with this one as well, so be creative.

We’ve talked about this subject several times before, but on pot B (the normal 250k tone control that operates from 0-5 on the knob), you can use any value of tone cap you want to achieve different tonal shades. On pot A, there is an 82k-ohm carbon-film resistor. As you turn the knob from 5 to 10, the added resistance reduces the effect of that resistor’s load on the pickups until it reaches 1 Meg, where it has almost no effect. Trying several resistor values and materials is another great adventure to be had. Personally, I like the value of the resistor to be 220k—give it a try. Notice that one end of both the resistor and the capacitor is soldered to ground on the TBX pot case.

The TBX tone control isn’t rocket science, but it is effective. The addition of this unique control can add some tonal options to your palette without altering the classic appearance of your Strat. Next month, we’ll talk about more possible mods for the TBX control. Until then, keep on modding.

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Dirk Wacker
Dirk Wacker lives in Germany and has been a guitar addict since age 5. He is fascinated by anything related to old Fender guitars and amps. He plays country, rockabilly, and surf music in two bands, works regularly as a studio musician, and writes for several guitar mags. He is also a hardcore DIY-er for guitars, amps, and stompboxes, and he runs an extensive webpage (singlecoil.com) on the subject.