Tech Bench - Mod Squad: Potent Pot Mod
By Ed Friedland
Wed, 4 Dec 2013
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Pot Mechanics: Moving the wiper farther from the “hot” side of the circuit increases the resisitance between lugs 1 and 2 and lowers the resisitance between lugs 2 and 3. The additional resistance between lugs 1 and 2 lowers the output.
There is no shortage of reasonably built, attractively finished import basses on the market these days, and for as little as $180, you can buy something you might actually enjoy playing. While these predominantly Asian imports can serve as a great starter bass (or even a “hazardous duty” axe for a pro), for some it is impossible to resist the pull of Turd Polishing Syndrome. “Oh man, this bass would be so much cooler with Nordstrand Pickups and a Mike Pope preamp!” they think. “And then I’ll get a Babicz bridge, Gotoh tuners, and refinish it in seafoam green.”

The primary indicator of T.P.S. is the subject’s belief that modifying a $180 bass with $820 worth of parts will somehow create a $1,000 instrument. Many folks happily play their creations for years; in fact, the number of online mega-threads dedicated to the humble Chinese “SX” (Essex) brand staggers the mind. But for some, this process leads to the inevitable “walk of shame” terminating in the for sale section, where the gritty reality of T.P.S. is learned: There is no recouping. Of course, for the modder-in-training, import basses are a great platform for experimentation, and in many cases simple, inexpensive mods can produce big improvements in overall performance. Before you drop $400 on new pickups, one of the least expensive ways to improve the sound of a cheapo is to upgrade the potentiometers, otherwise known as “pots.”

Pot Strains

A standard passive bass routes the signal from the pickup(s) to the volume pot, to the tone pot, and then to the output jack. What is found under the hood of a stock P-Bass looks so simple, it inspires even electronically challenged folks like myself to give it the old DIY try. While not hard to wire up correctly, there are some factors that could make swapping your pots a pot-ential nightmare. For starters, there are two types of pot commonly employed: linear taper and audio taper (sometimes called a logarithmic, or “log” pot). For general purposes, you’ll want audio taper, as it produces a smoother and more musical sweep from “0 to 10.” A linear pot will reach what sounds like full value abruptly—from “0 to 3,” for example, and you’ll hear little change for the rest of the sweep. However, this characteristic can be useful in other circumstances (below).

Pot Values

Pots come in different values, usually expressed in kilohms (kΩ, sometimes just read as “K”), a measure of resistance. A pot’s resistance affects the total load of the circuit in your bass, influencing the tone in somewhat predictable ways. As you increase the resistance of the pot (and so, the entire circuit), the bandwidth increases as well, accentuating high frequencies. For example, a standard setup on a J-Bass with single-coils employs 250kΩ pots for volume, volume, and tone. The darker effect of a 250K pot is meant to balance the inherent brightness of most single-coil pickups. With humbuckers, 500K pots are recommended, as their brighter response counterbalances the decrease in high frequency typical of heavily wound dual-coil pickups. Although hum-canceling, a split-coil P-Bass pickup has only one coil under each string, making 250K pots the stock choice. If you want to upgrade your instrument while preserving the basic character, use better-quality pots of the original value— when you ditch the schwag and upgrade to primo like Alpha, Bourn, or the ubiquitous CTS pots, there will be a distinct improvement in clarity, and a more “solid” feel to the rotation.

Cooking With Pots

Pots of specific values can be seen as part of a tone recipe. Putting a 500kΩ pot into a P-Bass can have several results. As a volume pot, 500K will effectively brighten the tone and provide a bit more output. However, it can also cause an impractical drop in volume from “10 to 8”—a problem that can be mitigated by the use of a linear pot. A 500kΩ tone pot will extend the high-frequency output as well, but rolling back on the control will still produce the response of a 250K pot. The pickup has a profound influence in the total circuit—something to consider when thinking about pot values. For example, many people use 500K for one or both pots with “hot” P-style pickups like the Seymour Duncan Quarter Pounder or DiMarzio DP146. The inherently stronger midrange of these pickups can benefit from the increased highs a 500K pot can induce, making it a common mod for players using heavy effects and overdrive. Using a 1MΩ (megohm) pot can get extremely bright, and in the volume position, may be too much for anything but a super dark pickup. But many pickups can benefit from the extended high end of a 1MΩ tone pot, while retaining the ability to roll it off.

What Size Pot?

On a more mundane level, there are some measurements you’ll want to be aware of. First, there are regular pots, and mini pots; figure out which you’ll need. A measurement worth checking is the mounting hole size: ⅜" is standard, but not always with imports. Whether you’re buying new knobs, or reusing the old ones, you’ll want to note whether they require split-shaft pots with a 6mm shaft diameter, or solid shaft—typically ¼" diameter. The height of the shaft is also worth noting; most pots have a ⅜" shaft, but the threading area beneath it can be ⅜" or ¾" in the case of a long-shaft model. If you buy the wrong height, you may have trouble installing the pots correctly.

Bust A Cap

You may have noticed a small capacitor wired between the hot terminal and the casing of the tone pot (ground). The capacitor is what turns the pot into a tone control, by “bleeding” the highs to ground, creating a variable lowpass filter. Even though the signal at the cap doesn’t make its way to your output jack, its presence in the circuit makes it a part of the tone recipe. Typically in a passive bass, you’ll see a cap with a value of .047μF (microfarads). While different-brand caps of the same value have distinct characteristics, their actual values can vary ±20% or more, making it tough to pick a predictable winner—but a popular choice is the Sprague “Orange Drop” capacitor, found for many years in Fender and Gibson instruments. The real fun begins when trying caps with different values: Lowering the value sets a higher corner frequency (where the high rolloff begins), removing less of the highs as you rotate to “0.” But another effect is the shift in the resonant peak frequency; higher-value caps lower the resonant peak, which is most noticeable with the tone control completely off. If you’re looking for dark, punchy tone, a .047μF cap rolled back will put you in the ballpark. If you want to preserve more mids and highs as you roll back the tone, look into lower-value caps.

As you can see, even the seemingly simple task of swapping pots can be fertile ground for geekery of immense proportions … and it gets way deeper. But with this general information, you can begin to form your own ideas about tone recipe, and at very least, not buy the wrong part! When swapping out the guts of your axe, remember that no single element is the magic bullet; you are dealing with an integrated system where all parts contribute to the result. A wide tone palette is possible, even with simple passive electronics—but if you’re cooking up your own recipe, don’t forget the pot!

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