Have you ever watched the concert videos of some classic rock band? I still remember the first time that I saw Led Zeppelin’s “The Song Remains The Same”. Wow, Jimmy Page really was the embodiment of a guitar God, wasn’t he? When I started playing electric guitar, I was in awe at how he could dial in any number of tones just by turning the knobs on his Les Paul. No sir, he wasn’t touching his amps or pedals every twenty seconds, just his guitar. It took a fair bit of playing before I learned to use the knobs on my guitars, but eventually I was able to go from a shimmering clean sound to an aggressive distortion just by turning the volume knob and playing harder. So how do these magic “knobs” work?
First of all, the knob is only the decorative and tactile cap on top of the device that does the real work. Underneath is something called a potentiometer, pot for short. Potentiometers are electrical components that behave much like a variable resistor. On the outside, they have a spinning shaft and three terminals. Within their casing, they have a ring of a resistive carbon compound and a wiper that’s attached mechanically to the shaft, and electrically to the middle terminal. As you turn the shaft, the wiper will move across the resistive compound, increasing the resistance between one of the outside terminals and the middle terminal. At the same time, the resistance between the other outside terminal and the middle terminal will decrease accordingly to the pot’s taper. This means that the sum of the resistances between the middle terminal and the outer terminals will always be the maximum resistance of the pot. That’s all fine and good, but how do they work in a guitar? That depends on how they’re used. They are commonly used as volume and tone controls, and while the principles are similar, each use has some variables.
If x is the resistance from one terminal to the middle, y is the resistance from the other terminal to the middle, and z is the maximum resistance of the pot, then:
x + y = z
Pots as Volume Controls
Sure, it would be real easy to feed the signal from your pickup directly to your amp, but isn’t it better to have some control over your sound? The most basic way of using a pot in a guitar is as a volume control. Basically, you feed your pickup into the pot, have one terminal send the signal further down the chain (possibly, though not normally, to tone pots or to output jacks), and have the other grounded. The idea is, without any extra resistance, the signal will naturally want to ground itself. As you increase the resistance to ground, more and more of the signal will continue down the rest of your chain. You may notice that, as you turn the volume down, you lose some of the higher frequencies. All electrical components have a small amount of natural capacitance. One of the side-effects of this is that all frequencies will not be effected uniformly by resistance. Higher frequencies will be the first to short to ground, and this is the principle applied in tone controls.
Pots as Tone Controls
It can be helpful to look at tone controls as the inverse of volume controls in function. Where with volume controls you are actually decreasing the signal that’s allowed to pass down the circuit, with a tone control you’re increasing the amount of LOSS to the higher frequencies. The idea is, you connect the tone pot in parallel with the volume pot with a capacitor, and connect another terminal to ground. When the tone pot is turned so that there is maximum resistance across the capacitor, almost no signal is lost, and the effect is nearly negligible. When the resistance decreases, more and more signal is allowed to pass through the capacitor and short to ground. Remember how capacitors effect different frequencies in different ways? In effect, a capacitor is a large resistor for lower frequencies, and provides almost no resistance at all for higher frequencies. This means, that as you turn your tone down, the higher frequencies will be shorted to ground much faster than the lower frequencies, which will pass relatively unhindered to your output jack. Now, there are other ways to install a tone control in a guitar, but the idea of using a capacitor to ground high frequencies is the same.
Now that’s the basic idea behind volume and tone controls. I’m not going to bother with wiring diagrams, because I’m not trying to describe how to install them, but rather what they do. Now, let’s say you have to replace a pot! There’s a lot of different kinds available. How does the taper need to behave? How resistive does it need to be? How long does the shaft need to be?
Audio Taper and Linear Taper
The taper of a pot refers to the rate at which the resistance changes as you turn the knob. Look at your guitar amp. Have you noticed that most of the volume is either in the first quarter turn of the knob, or the last quarter turn of the knob? That’s because the volume control is designed using an Audio Taper, or logarithmic pot. It does not respond even across its range of motion. This is the most common type of pot used to volume controls, because the taper corresponds to the way that our ears perceive volume, which is not even.
A Linear Taper pot responds evenly across its whole range of motion. At half-way, resistance will be half, at 3/4, the corresponding resistance will be 3/4 (while the other terminal will be 1/4). The graph of the relationship is a straight line. Many people like a Linear Taper tone control, but this is mostly a matter of taste. By and large, the most common taper used in guitars (both volume and tone controls) is the Audio Taper.
I could easily dedicate an entire article to the topic of potentiometer resistances. This is the single potentiometer variable that will most effect your tone. What resistance you use will largely be a matter of taste, but there are a few “starting points” depending on what kind of pickups you have.
If you have lower-impedance pickups (ie. low-output single coils) you’re probably going to be looking at pots in the 100,000 ohm range. If you find that your guitar’s tone sounds muffled and squashed, you can try 250k ohm pots, but you may find your sound turning much too brittle.
Medium impedance pickups (ie. high output single coils & low output humbuckers) generally sound best with 250k – 330k ohm pots. With some, you may be able to justify 500k ohm pots, but again… Depending on your pickups and taste, you might find that too brittle.
Higher impedance pickups (ie. modern humbuckers) can scrape by with 300k ohm pots if you like the dark tone, but you’ll get much more clarity at 500k ohms. This is especially the case with modern pickups that are designed to be “hotter” versions of Gibson’s “Patent Applied For” humbuckers of the late 1950s.
Very high impedance pickups (ie. modern metal passive humbuckers) generally behave well with 500k ohm pots, but you might find that 1,000,000 ohm (1M ohm) pots work for you.
You can also experiment in using different resistances for your volume pots and tone pots. The above applies primarily to volume pots, but since tone pots are normally wired in parallel to volume pots, their resistance counts too. In effect, if you made the capacitor a regular wire, a 500k volume pot and a 500k tone pot would provide a resistance of 250k ohms. For this reason, some people actually cut their tone pots right out of their circuit, and leave only the volume pot to provide a full 500k ohms for very bright (possibly harsh and brittle) tones.
If v is the resistance of the volume pot, t is the resistance of the tone pot, and R is the total resistance, then in parallel:
1/v + 1/t = 1/R
There’s not much to say about the shaft length of a potentiometer. What do you need it to stick through? On a Strat, where you only have to stick through the thin material of the pickguard, you don’t need a long shaft. On a Les Paul, you have a fat piece of carved maple to stick through, and you’ll need something longer.
Remember when I was rambling about Jimmy Page at the start? He achieved much of his tonal depth by using a push-pull potentiometer. Basically, they have several banks of terminals, and allow you to use them in different configurations. You switch between configurations by pushing or pulling the shaft. Using push-pull pots you can tap individual coils from a humbucker or switch between parallel and series pickup configurations.
Make It Work For You
The potentiometers are just the gateway to all kinds of fun experiments with a guitar. People modify their pots with nail polish, cut them right out of the circuit, wire them together in different way… Any interesting configuration that works, you can bet people have tried, all in the search for that holy grail of tone. People look for vintage bumblebee capacitors to go with their 500k pots, and buy pickups that they can coil tap with their push-pulls. You can get caught up in all the racket very easily, trying to find the perfect electrical components to brighten up your sound, or mellow it out. Let the music flow from your fingers, and I know you’ll be impressed, no matter what’s in your axe.
On our next outing, I’d like to look into something that’s useful for any guitarist to know. Keeping your fingerboard clean, buttery, and beautiful can be a stressful job, especially when you live in dry or humid climates. If you don’t clean it, it gets covered in grime… That doesn’t help your playing any. If you clean that grime off, it may dry out. At best, a dry fingerboard is just pale and ugly. At worst, it may begin to crack or chip, or your frets may go loose! We’re going to take a look at how to keep your fingerboard, rosewood or maple, in shape.
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