Potentiometers are small and cute, but it pays to know their anatomy.
If you're a Trekkie like me, you instantly know what my headline refers to: “The Trouble with Tribbles," episode #44 from the original Star Trek's 1967 season. In our shop, we call potentiometers (“pots") tribbles because those cute little rascals can cause enormous trouble. So here are some guidelines to help you avoid ending up like Captain James T. Kirk in this iconic episode.
Almost all electric guitars have one or more pots to control volume and tone, or for such special uses as blending two pickups. Unfortunately, it's usually not a question of whether a pot will fail—only when. Pot problems can range from crackling to non-responsive controls, or even a dead guitar. Sometimes noisy pots can be cleaned or repaired, but often the only remedy—especially with cheap factory pots—is to change them.
Changing a pot isn't difficult—it's usually just a matter of locating an identical part and duplicating the original solder connection. But I get at least five emails a day from guitarists who aren't sure how to determine the correct part for their needs. We'll shed some light on that, but first, let's look at the parts that make up a pot.
Pot anatomy. Let's be sure we're using the same terminology. Photo 1 shows the main parts of a typical pot. As you can see, the part names are far from standardized.
1. The casing (also called housing, case, body, or enclosure).
2: The bushing (also called threaded collar or shank).
3: The shaft (occasionally called top).
There are other parts (terminals, baseplate, nut, washer, etc.), but let's focus on these three main ones. When seeking a replacement pot, sometimes the guitar model name is all you need. For example, searching “vintage Strat" will probably locate the correct pots. But in other cases, you must know additional parameters, namely: measurement type (U.S. inch vs. metric), nominal resistance, casing diameter, bushing length and diameter, and shaft type and diameter.
Parallel universes. Most U.S.-made guitars (like Fender, Gibson, and PRS, to name a few) use U.S. pots measured in inches. Meanwhile, most guitars produced outside the U.S. (Epiphone and Ibanez, for example) employ metric pots. Some companies (like leading American pot manufacturer CTS) only make U.S. inch pots, while other brands, such as Alpha and Bourns, build both types, so be careful when choosing replacements.
Nominal resistance.This is often referred to as the pot's “value," with 250k, 500k and 1M as the standards for passive guitars, while and pots with values between 10k and 50k are common in active circuitry. The resistance is usually stamped on the casing or baseplate. If you can't find the info, remove all wires from the pot and test it with a standard digital multimeter (DMM) set to resistance. You may also be able to identify the pot value from a wiring diagram on the manufacturer's website.
Casing diameter. Usually only two pot sizes are used in guitars: big ones (standard) and small ones. The standard big size is .937" (approximately 23.8 mm) and 24 mm for metric pots. The small size is .650" inches (about 16.5 mm) and 16 mm for metric. Your measurements can indicate whether you have a metric or U.S. inches pot.
Bushing length and diameter. There are three common bushing lengths, often referred to as short, semi, and long. Short pots are usually installed through a pickguard or metal control plate, as on Strats and Teles. Their size is .250" (about 6.35 mm), and 6 or 6.5 mm in metric. Semi pots are for installation through wood, as on an SG or PRS. These are .375" inches, or 3/8" (roughly 9.5 mm), and 10 mm in the metric version. Long pots are used with thick wooden tops, like on a Les Paul. These are .750" (about 19.05 mm) and 20 mm in the metric version.
Luckily, there are only two common bushing diameters: 3/8" (about 9.5 mm), with a 10 mm metric equivalent. A thinner version (often found on pots with the smaller casing) is 0.31" (8 mm). The metric versions are 8 mm as well.
Shaft type and diameter. There are two shaft types: split-shaft (for push-on knobs, as on a Stratocaster) and solid-shaft for knobs with a set screw (like a Telecaster's dome knobs). On U.S. inch pots, the typical split shaft's diameter is .236" (about 5.95 mm) with 24 “fine-knurled" teeth, and the metric version is 6 mm with 18 “coarse knurled" teeth. Careful when replacing push-on knobs for split-shafted pots—you can't push knobs made for 18 teeth onto shafts with 24 teeth, and vice versa. (You can fine-tune the fit by squeezing the shafts together slightly or spreading them further apart.) A U.S. inch solid shaft is .250" (approx. 6.35 mm) while the metric version is 6 mm.
Pot-mounting hardware. Finally, to install pots into a guitar or pickguard, just follow these steps: Remove all the hardware. If you need to adjust the height of the pot in the pickguard or the guitar, thread on a spare hex nut to use as a spacer. Then install the lock washer. Insert the pot into the pickguard or guitar. Place the washer on the bushing. Secure the pot with a hex nut. That's it!
Next month we'll explore a cool mod for Les Paul/SG/335 lovers. Until then, live long and prosper ... and keep on modding!
Looking for more great gear for the guitar player in your life (yourself included!)? Check out this year's Holiday Gear Finds!
D'Addario XPND Pedalboard
DR-05X Stereo Handheld Recorder
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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.