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A guitar knob is actually an electrical component called a potentiometer. The potentiometer is a simple passive device that is sort of like a variable resistor. A resistor causes signals to “drop” across it. So, for example, given a signal source and a single resistor, all of the signal “drops” across the resistor. If you had two resistors in series, part of the signal “drops” across the first resistor, then the rest of the signal “drops” across the second. This configuration of two resistors in series with the output between the two resistors is called a voltage divider. It is a clever way to “turn down” any signal source.
Potentiometers (“pots”) are basically voltage dividers that sweep the two resistance values. The outer pins are the outer legs of the resistors, and the middle pin is the point between the two resistors. To match a schematic up with real-life, when you are looking at the middle pin pointing upward on the schematic, it is like looking at the real potentiometer from the top where the shaft is pointing at you. When the pot is turned all the way clockwise, the right and middle pins are connected together, and all the resistance is between the middle and left pin. When the pot is set to the middle position, equal resistance is between the outer pins and the middle pin. And when turned all the way counter-clockwise, the resistance lies between the right and middle pin, and the middle and left pin are directly connected. Often when knobs are accidentally wired “backwards”, it is usually because the outer pins were confused. Remembering your orientation will help prevent this.
There are basically only two uses of a potentiometer: a variable voltage divider, and a variable resistor. The variable voltage divider is most often used for volume knobs. Recall that voltage dividers “turn down” signals. When we connect our signal source to the right pin, the ground to the left, and the rest of the circuit to the middle, the output will see either the full signal, part of the signal, or the ground depending on where the potentiometer is set.
In this configuration, the actual value of the potentiometer has a small effect on the tone of the signal. These reasons will make more sense when we get to tone circuits, but basically the amount of resistance in one part of a circuit will affect the performance of earlier and later non-isolated tone circuits. The general rule-of-thumb is that higher value pots are brighter and lower value pots are darker.
The second use of a potentiometer is as a variable resistor. This is when you either disconnect one of the outer legs of a pot, or you short it to the middle pin. Either is fine. When the pot is turned one way, the full resistance is seen at the output, or when the pot is turned the other way, the resistance is bypassed. The shorted/disconnected leg does not affect the circuit. The variable resistor is most often used as a Tone control. The variable resistor controls how much effect a tone circuit has on the signal. Again, its value affects the tone of the guitar, but these reasons will become easier to understand once we study tone circuits. For now, just imagine the variable resistor as getting “in the way” of a tone circuit. The higher the value, the more “in the way” it gets and the Tone circuit will have less effect; the lower the value, the less “in the way” the pot is and the tone circuit will have more effect.
The best way to learn how these pots are used is to study their use in circuits and learn a little more about how tone circuits work. Luckily, these topics are right up next.