The electric guitar's volume pot is a simple device, and some would think there's not much you can do to hotrod things - or even that there's not much point to modding it. However, it does affect the tone of a guitar and you can do a few very simple tweaks to alter how it sounds and how it responds to volume changes.

This month let's have a look at the pots themselves, as understanding them allows us to tweak things effectively.

Electric Guitar Volume Pots 1: Basics
See also: Electric Guitar Volume Pots 2: Treble Bleed Mods

What are Pots?
"Pot" is the abbreviation for potentiometer, and they are simple variable resistors whose resistance value is changed by turning a knob. If you take a look at the schematic symbols for a resistor (see below) and for a pot (ditto) you will see that they are very similar, with just an added contact on the pot which is known as the wiper.

Schematic symbol for a resistor Schematic symbol for a pot A typical guitar pot How the schematic relates to the pot The terminals and the track inside a pot

Pot Construction
The outer terminals, known as the leads, are connected inside the pot by a "track" of a resistive material, which forms a resistor betweeen them (the value of which is the value of the pot's resistance). The wiper, being able to move anywhere along the track, can tap into anything from zero to the full resistance of the pot.

Resistance Values
Pots are measured in Ohms and Kilohms (K). The standard values used for guitars are 250K or 500K. The general rule is that 250K is used on guitars with single coil pickups (like Fender Strats), while 500K is used for humbucker equipped guitars (like Gibson Les Pauls). If you have a guitar with a mix of both types: HSS or HSH (where H is Humbucker and S is singlecoil), use 500K.

The resistance of the volume pot is crucial to pickup performance. Higher value pots cause the pickups to resonate at a higher frequency, which results in slightly higher output and presence with the result that the sound is noticeably brighter. Lower value pots do the reverse make the sound darker, with less prescence and lower output.

It's important to note that a lot of pots will measure differently from the value at which they are rated. While the tolerance of a pot (the percentage variation from rated value) is usually within 20%, which is fine, but within 10% is much better. It is also better to have a pot which measures higher than it's value, than one which is lower, as a good sounding pickup can be muffled by a pot that measures too low. Some specialist guitar parts suppliers will pre-test pots to ensure they are well within acceptable range, but this is rare and expensive.

Measuring resistance
To measure the real value of a pot, simply test the two outer leads of the pot with a multimeter (when it is not connected!).

Taper
The taper of a pot (sometimes called the law), is how the resistance changes as you turn the knob. There are two tapers used: linear and logarithmic (log) - the latter also called left handed or audio taper. With linear pots the resistance changes evenly as you turn the pot - every 10% of the pots rotation will change the resistance by 10%. Log pots were designed to compensate for a particular oddity with human hearing: the human ear does not respond linearly to loudness. It responds to the logarithm of loudness. That means that for a sound to seem twice as loud, it has to be almost ten times the change in air pressure (which is also why the decibel scale of loudness is logarithmic). So, as you turn a log pot up, the resistance change gets faster and faster, which your ears hear as a smooth gradual volume change.

Thanks to the logarithmic nature of our loudness perception, we traditionally use log taper pots for volume controls and linear for tone - although it should be noted that some people do like like one or the other. Unfortunately the laws (sorry) of economics and the curse of the MBA, make it cheaper for a manufacturer to use all of one type of pot, some go as far as So if you find your volume comes up dramatically in the first half of pot rotation, and then there is very little change in the last half - you have a linear pot instead of a log. Also, if your tone control seems concentrated in the last 10% of the rotation, you need to replace the log pot with a linear model

Measuring Taper
If you have an unknown pot, you can figure out what taper it is. You measure the resistance across the outer terminals (which gives you the total resistance), then turn the pot to half its rotation and measure the resistance from one of the outer terminals to the wiper. If the resistance is 50% of the total resistance, then the pot is linear. If you measure only 10% to 20% of the total resistance and 80%-90% to the other outer lug, the pot is an audio taper.

Right-handed Taper for Leftys
Of course, audio taper pots do not cater for the sinister guitarists of the world (what else is new?). There used to be a third type of taper - the antilog or right-handed taper. While not designed with left handed guitarists in mind, they were useful for this purpose. Unfortunately they are no longer made (economics again, apparently), but there is a way to turn a linear pot into a log one by adding a resistor. Taking this trick further lets us do is make a proper two pickup balance control, using a dual pot to give a log/antilog taper.

In Use
In the simplest possible guitar application, you use the following circuit.

Next month, I'll look at ways to tweak the performance of your volume pots, including changing values, treble bleed mods, antilog mods and more. Article Here