Premier Guitar

The Fender TBX Tone Control Mod, Part 2

October 20, 2010
 
Replacing the TBX’s stock resistor with a second capacitor will give you greater control over the amount of bass cut in the circuit.

Swap a 220 kΩ resistor for the TBX’s stock 82 kΩ shunt resistor for smoother operation in passive setups.
After talking about the basics of the Fender TBX tone control last month, let’s now take a closer look at some of the mods you can make to this system.

First, a few refreshers. Remember that the values of the two pots are fixed, so you can’t experiment with them. I also do not suggest opening the TBX’s dual-ganged pot—the system is very tricky, and there are no mod opportunities inside the pot. Also keep in mind that the TBX tone control is not a treble booster or anything like that—you cannot increase anything with passive electronics. The system works by allowing you to reshape the tone by de-emphasizing certain frequencies, thus making others more prominent. The use of inductors (which is what a pickup behaves like in a guitar circuit) and capacitors can create resonant peaks and valleys, further coloring the overall tone. Some people like this interaction, while others don’t, but it’s all relative and it all works at unity gain.

Experimenting with Tonecaps
The bottom pot (with the shaft up) of our TBX system is pot B on the drawing and the normal tone control we all know—a high-pass filter. It’s a standard 250k audio pot with a range from 0 to 5 on the knob. So you could say it is a normal passive tone control, but with half of the rotation. The 0.022 uF tonecap connected to this pot works like a standard tonecap, meaning there are tons of options here. Try other values from 2200 pF up to 0.1 uF, try different types of caps like metal-film, paper-in-oil, paper waxed, or silver mica, or experiment with used or new-old-stock caps. Your choices are virtually unlimited, so be brave and go wild.

Experimenting with Resistor Values

On pot A, there is an 82 kΩ carbon-film resistor. This is another area ripe for experimentation. The value of the resistor influences the center frequency and the amount of bass cut out of the circuit (it’s a low-pass filter). I like mine to be between 180-220 kΩ in Fender-style guitars, as these values provide a smooth and natural bass cut. Try values from 20-500 kΩ and see what you like best.

You can also experiment with different materials for this resistor, such as metal-film or carbon composition models. Some customers report they get the best results with NOS Allen-Bradley carbon composition (CC) resistors. These resistors have less background noise and were the “gold standard” back in the ’50s and ’60s for all Fender amps—not a shabby point of reference!

There are more mods you can do to the TBX tone control, such as using a second capacitor instead of a resistor to rewire the TBX control for more bass cut, or wiring the second end of the resistor connected to pot A to the same lug as the capacitor on pot B (instead of connecting it to ground) for a more even and natural effect in passive guitars. Let’s take a closer look at these options.

Rewiring for More Bass Cut
You accomplish this by replacing the stock resistor with a second capacitor, which will act as a bass-cut capacitor and determine the bass-cut frequency. The treble-cut capacitor from pot B stays untouched and determines the treble-cut frequency. As we discussed earlier, you can try different values and cap types here as well. As a basic rule, the higher the value, the lower the frequency and the greater the effect. (Likewise, the lower the value, the higher the frequency and the less the effect.) Instead of the resistor on pot A, connect a second capacitor as shown in the diagram below. Start with a value of 2200 pF and work your way up to 4700 pF, or even 6800 pF for a radical effect. Personally, I like the bass-cut cap to be 10 percent of the value of the treble-cut cap, so if you use a standard 0.022 uF tonecap on pot B, try a 2200 pF bass-cut cap on pot A.

Rewiring for Smoother Passive Operation

On the stock TBX control, the 82 kΩ resistor is always in the circuit, working as a shunt, no matter where the knob is set—even with the pot fully opened. The TBX control was originally developed to work with active circuitry, such as the Eric Clapton Mid Boost, where an 82 kΩ resistor has no deleterious effect on the tone. But an 82 kΩ shunt resistor is something you don’t want in a guitar with high-impedance passive circuitry. After the Clapton signature Strat, Fender included the TBX control in passive guitars, and the 82 kΩ resistor stayed there untouched—heaven knows why.

The TBX control is very useful in passive systems if you modify it slightly, as shown here. Remove the tone-sucking 82 kΩ resistor, add a jumper between two of the pot’s lugs, and add a 220 kΩ resistor. The new resistor is necessary to make the transition between the two pots at mid-position seamless, with no abrupt change in tone or apparent volume as the circuit is handed off from one pot to the other. A lot of pro players prefer this slightly modded version over the stock configuration.

All right, that’s it for this month. I hope you’ll have some fun with these mods. Next month, we will talk about Fender’s active Eric Clapton Mid Boost circuit, and discuss how you can build your own budget version without breaking the bank. Until then, keep on modding!


Dirk Wacker
Dirk Wacker lives in Germany and has been addicted to all kinds of guitars since the age of 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 session musician for a local studio, 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.