One of Fender''s oldest factory mods features a stacked potentiometer
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.
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.
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Created in collaboration with legendary guitarist George Lynch of Dokken and Lynch Mob fame, the Mr.Scary Mod adds an adjustable tube gain stage and an onboard Deep control, which together are designed to enable an amp to have increased sustain while still retaining note definition and dynamics.
LegendaryTones, LLC today announced production availability of its new Mr. Scary Mod, a 100% pure tube module designed to instantly and easily expand the capabilities of many classic amplifiers with additional gain and tone shaping. Created in collaboration with legendary guitarist George Lynch of Dokken and Lynch Mob fame, the Mr.Scary Mod adds an adjustable tube gain stage and an onboard Deep control, which together are designed to enable an amp to have increased sustain while still retaining note definition and dynamics.
Originally released as the Lynch Mod in February 2021, the updated Mr. Scary Mod features the same core circuit as the Lynch Mod but is now equipped with a revised tube mix combo per George’s preference as well as a facelift in a newly redesigned electro-galvanized steel enclosure. As with the Lynch Mod, each run will be limited and the first run in Pumpkin Orange with Black hardware is limited to just 150 pieces worldwide.
The Mr. Scary Mod adds an adjustable tube gain stage on top of the cathode follower position, keeping note definition and articulation while further increasing sustain. Each Mr. Scary mod is meticulously built by hand in the USA, one at a time, and tuned using high-grade components. Equipped with a single ECC81 (12AT7) in the first position and ECC83 (12AX7) in the second, the Mr. Scary Mod can clean up beautifully when rolling down your guitar’s volume, and still adds scorching gain when you roll it back up. This is a gain stage that’s been tuned and approved by the ears of the maestro George Lynch himself.
“The Mr. Scary Mod excels with dynamics and is incredibly touch-responsive, allowing me to shift from playing clear, lightly compressed cleans to full-out aggressive sustain and distortion –and control it all simply by varying my guitar’s volume control and picking,” said GeorgeLynch. “In many ways, it’s an old-school approach, but it’s also so much more natural and expressive in addition to being musically fulfilling when you can play both the guitar and amp dynamically together this way.”
The Mr. Scary Mod installs in minutes, is safe and effective to use, and requires no special tools or re-biasing of the amplifier. Simply insert the module into the cathode follower preamp position of compatible amplifiers (includes Marshall 2203/2204/1959/1987 circuits) and
immediately get the benefit of enjoying a hot-rodded amp that delivers all the pure harmonic character that comes with an added pure tube gain stage. The handmade in the USA Mr. Scary Mod is now available to order for $319.
For more information, please visit legendarytones.com.
October Audio has miniaturized their NVMBR Gain pedal to create two mini versions of this beautifully organic-sounding circuit – including an always-on gain device.
The NVMBR Gain is a nonlinear amp that transitions gracefully from clean boost to overdriven tones. Volume increases from just over unity to about 10db before soft-clipping drive appears for another 5db of boost. Its extraordinary ease of use is matched by outstanding versatility: you can use it as a clean boost, push a stubborn amp into overdrive or create a just-breaking-up sound at any amp volume.
October Audio’s new family of mini NVMBR Gain pedals includes a switchable version that allows you to bypass the effect: one option features brand logo pedal graphics, while the other sports a fun “Witch Finger” graphic with a Davies knob as the“fingernail”.
The second version in the new lineup is an always-on device featuring the Witch Finger graphic and Davies knob, with the same NVMBR Gain circuit that lies at the core of the switchable version.
- Knob controls gain and clipping simultaneously
- Stunning silver hammertone finish
- Switchable versions are true-bypass, available with classic or witch finger graphics
- Authentic Davies knobs, including the “fingernail”
- 9V center negative power supply required
- Dimensions: 3.63 x 1.50 x 1.88 in
Witch Finger (always on NVMBR Gain) demo
All October Audio pedals are assembled in Richmond, VA, and available for purchase directly through the online shop. Street price is $109 for NVMBR Gain footswitch versions and $89 for the always-on device.
For more information, please visit octoberaudio.com.