Experimenting with tonecaps and resistors on the TBX tone control
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
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.