How Electric Guitars Work
Part 3 – Capacitors
Capacitors allow much more of the high frequency sound to escape to ground. Cap’s are like a filter: they let the highest frequencies pass, but resist lower frequencies. Connected to the signal and ground wires as shown here, a capacitor would have a big impact on tone (reducing treble), but that impact would be uncontrolled.
Now if we add a control pot, we have a tone control: it can dial-in the amount of the signal that gets to the capacitor, so the cap’s treble-removing effect is variable. As you open up the pot, more signal goes to the cap.
To reverse the effect of your tone pot, you could rewire your guitar as shown below. Here a capacitor bridges the input and output lugs on a volume pot; this allows the high frequencies to bypass the pot and stay in the signal. Lower frequencies are blocked by the cap, and are controlled by the pot.