MOD GARAGE
DIRK WACKER
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 ver-
sions 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 pro-
vided 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-contain-ing 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.
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.
