Experimenting with tonecaps and resistors on the TBX tone control


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.

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