The tone control of an electric guitar allows the player to change the sound of the guitar from bright and thin to warm and full with the simple turn of a knob. How is that possible? First let's consider how an electric guitar actually makes sound the amplifier will pass along to our ears.
Everything starts with the strings being plucked, strummed or picked which causes the strings to move (vibrate). If you take metal strings and place them within a magnetic field, the mechanical energy of the strings will induce a voltage which drives a small electrical current. The pickups are usually just a coil of wire wrapped around a magnet or steel pole pieces near a magnet. So now the mechanical energy of the moving strings has been converted to electrical energy we'll call the "signal". To keep things simple let's just say the strings and pickups will produce a blend of low, mid and high frequencies.
When we use a volume control, we're manipulating all of the frequencies within the electrical signal. We hear the change as "loud" when the volume is turned all the way up, "quieter" as we turn the volume down and eventually "off" when the potentiometer is turned all the way down. With a tone control we can eliminate just the high frequencies with the addition of a capacitor.
In simplest terms a capacitor is made up of two conductors separated by an insulator (also known as the dielectric). The conductors are capable of holding a static charge and depending on the size, thickness and distance between the conductors it will have a certain amount of capacitance measured in "Farads", a term named after Michael Faraday (1791-1867). In reality one Farad is an enormous amount of capacitance so it is broken down into smaller units making it more practical. The most common value in electric guitars is microfarads (uf) and the two most common values are .022uf and .047uf. The higher the value the more high frequencies will be passed to ground making it sound warmer, fuller or bass heavy.
The tone control in an electric guitar uses the capacitor to filter out high frequencies and send them to ground. So where the volume takes all frequencies and sends them to ground, the tone control sifts out only the high frequencies. When the tone control is turned all the way "up" nothing is actually happening. The tone control is not actually making your guitar brighter, it begins to function as you start turning it down creating a pathway for the guitar signal. Once the signal gets to the capacitor only the high frequency can get through and when you solder the capacitor to ground those frequencies disappear.
There are a lot of opinions about which capacitors sound the "best" but in reality each person hears things differently and will have their own preferences when it comes to the sound of a guitar. Whenever a customer has asked about capacitors I usually start with the concept of how much high end they want to be able to filter out which will help decide which value to choose. If you are considering making a change to your electric guitar check out capacitor videos on YouTube to see what you might like the best. A $40.00 capacitor does not necessarily sound better but if a person spends $40.00 they'll almost certainly believe it does. The more playing experience you have the better you can hear subtle differences from one capacitor to another and it certainly can make a difference. However, be careful because it won't be long before you begin the race for better tone which is a race without a finish line.