Basic Electric Guitar Circuits: Potentiometers and Tone Capacitors

What is a Potentiometer?

Potentiometers, or “pots” for short, are used for volume and tone control in electric guitars.  They allow us to alter the electrical resistance in a circuit at the turn of a knob.

It’s useful to know the fundamental relationship between voltage, current and resistance known as Ohm’s Law when understanding how electric guitar circuits work.  The guitar pickups provide the voltage and current source, while the potentiometers provide the resistance.  From Ohm’s Law we can see how increasing resistance decreases the flow of current through a circuit, while decreasing the resistance increases the current flow.  If two circuit paths are provided from a common voltage source, more current will flow through the path of least resistance.

We can visualize the operation of a potentiometer from the drawing above.  Imagine a resistive track connected from terminal 1 to 3 of the pot.  Terminal 2 is connected to a wiper that sweeps along the resistive track when the potentiometer shaft is rotated from 0° to 300°.  This changes the resistance from terminals 1 to 2 and 2 to 3 simultaneously, while the resistance from terminal 1 to 3 remains the same.  As the resistance from terminal 1 to 2 increases, the resistance from terminal 2 to 3 decreases, and vice-versa.

Tone Control:  Variable Resistors & Tone Capacitors

Tone pots are connected using only terminals 1 and 2 for use as a variable resistor whose resistance increases with a clockwise shaft rotation.  The tone pot works in conjunction with the tone capacitor (“cap”) to serve as an adjustable high frequency drain for the signal produced by the pickups.  The tone pot’s resistance is the same for all signal frequencies; however, the capacitor has AC impedance which varies depending on both the signal frequency and the value of capacitance as shown in the equation below.  High frequencies see less impedance from the same capacitor than low frequencies.  The table below shows impedance calculations for three of the most common tone cap values at a low frequency (100 Hz) and a high frequency (5 kHz).

When the tone pot is set to its maximum resistance (e.g. 250k?), all of the frequencies (low and high) have a relatively high path of resistance to ground.  As we reduce the resistance of the tone pot to 0?, the impedance of the capacitor has more of an impact and we gradually lose more high frequencies to ground through the tone circuit.  If we use a higher value capacitor, we lose more high frequencies and get a darker, fatter sound than if we use a lower value.

Volume Control:  Variable Voltage Dividers

Volume pots are connected using all three terminals in a way that provides a variable voltage divider for the signal from the pickups.  The voltage produced by the pickups (input voltage) is connected between the volume pot terminals 1 and 3, while the guitar’s output jack (output voltage) is connected between terminals 1 and 2.  From the voltage divider equation below we can see that if R1 is 0? and R2 is 250k?, then the output voltage will be equal to the input voltage (full volume).  If R1 is 250k? and R2 is 0?, then the output voltage will be zero (no sound).

Potentiometer Taper

The taper of a potentiometer indicates how the output to input voltage ratio will change with respect to the shaft rotation.  The two taper curves below are examples of the two most common guitar pot tapers as they would be seen on a manufacturer’s data sheet.  The rotational travel refers to turning the potentiometer shaft clockwise from 0° to 300° as in the previous visual representation drawing.

How do you know when to use an audio or linear taper pot?

It’s really a matter of personal taste when it comes to volume control.  Notice how the rate of change is much more dramatic on the audio taper pot when traveling back from 100% to 50% rotation.  This means that the same amount of rotation would give you a more intense volume swell effect with an audio taper than with a linear taper.  Using a linear taper volume pot would give you a more gradual change in volume which might feel like you have more fine control with which to ease back the volume level.

For tone control, it’s basically standard practice to use an audio taper.  The effect of the tone circuit is not very noticeable until the resistance gets pretty low and you can get there quicker with an audio taper.

How do you know what value of potentiometer to use?

The actual value of the pot itself does not affect the input to output voltage ratio, but it does alter the peak frequency of the pickup.  If you want a brighter sound from your pickups, use a pot with a larger total resistance.  If you want a darker sound, use a smaller total resistance.  In general, 250K pots are used with single-coil pickups and 500K pots are used with humbucking pickups.

Specialized Pots

Potentiometers are used in all types of electronic products so it’s a good idea to look for potentiometers specifically designed to be used in electric guitars.  If you do a lot of volume swells, you’ll want to make sure the rotational torque of the shaft feels good to you and most pots designed specifically for guitar will have taken this into account.  When you start looking for guitar specific pots, you’ll also find specialty pots like push-pull pots, no-load pots and blend pots which are all great for getting creative and customizing your guitar once you understand how basic electric guitar circuits work.

Kurt Prange (BSEE) is the Sales Engineer for Amplified Parts (www.amplifiedparts.com) in Tempe, Arizona, United States.  Kurt began playing guitar at the age of nine in Kalamazoo, Michigan.  He is a guitar DIY’er and tube amp designer who enjoys helping other musicians along in the endless pursuit of tone.

Yellow Jacket Technical Information

Yellow Jackets  tube converters allow EL84 power tubes to be used in place of the most common guitar amp power tubes including 6L6, EL34, 6V6, 7027, 6550 and 7591. Most Yellow Jackets provide a substantial output power reduction and a “self-bias” Class A configuration for the EL84 so that no bias adjustment is required. Yellow Jackets are like getting a whole new amp. Read more

NAMM 2011: PRS Guitars electronics, hardware, neck shape and finish

NAMM 2011 — For more than 25 years, PRS Guitars’ culture of ingenuity and continual improvement has pushed their guitar manufacturing bar to new levels. The 2011 PRS Guitar product line exemplifies this culture with the execution of a thorough plan to enhance each instrument.

Innumerable changes have been put into action to improve the appearance, playability and tone of PRS instruments.Whether it’s the new 53/10 pickups, two-piece bridge, steel bridge components, new neck shapes, V12 finish or new PRS Phase III locking tuners, the amount of time and energy PRS has invested reviewing, enhancing and implementing high impact changes in their quest for tone is unprecedented, even by PRS standards — PRS Guitars. Read more

CE Distribution Sells MusiCap Capacitors

CE Distribution, LLC has been chosen as the exclusive distributor for MusiCap Capacitors. The world famous MusiCapTM film-and-foil capacitors (formerly Hovland MusiCaps), are made in the USA by UpTone Audio and are distinguishable by their new red, white, and blue colors.

Alex Crespi, owner of UpTone Audio, said, “CE Distribution was the best choice for MusiCapsTM because of their outstanding reputation for quality products, excellent customer service and quick, accurate shipping Read more

Jensen Jet Falcon 10” Instrument Speaker

Jensen Musical Instrument Speakers is pleased to announce the latest addition to the Jensen Jet series, the 10-inch Falcon. With its 40 watts of power, a ceramic magnet, and a seamed green cone made from all natural materials, the 10-inch Falcon is specially designed to ensure optimum tone for all styles of music.

The Falcon 10-inch tone is warm and straightforward with a pronounced low end. Its mid frequencies are slightly enhanced over the Falcon 12-inch, and it has a very sweet, yet cutting, high end. When presented with overdrive distortion it creates some mean fuzz.

This new Jensen speaker once again demonstrates Jensen’s tradition of giving players what they are asking for in tone. Watch for more Jensen Jet series speaker designs on the horizon.
jensentone.com

Jensen Jet Falcon Guitar Speakers

Yellow Jacket YJS Demo | Tube Guitar Amplifiers

Yellow Jacket YJS Demo for tube guitar amplifiers — Watch the video demonstration of the Yellow Jacket YJS vacuum tube converter for tube guitar amplifiers.

Read more

JJ Electronic releases new tubes!

CE Distribution, the primary distributor of JJ Electronic vacuum tubes, is pleased to announce the release of three new JJ tube types. JJ Electronic has been producing premium vacuum tubes in the Slovak Republic for over 16 years. JJ products are known in the industry as the best sounding and most reliable tubes produced today.

The three new tubes are all very welcome additions to the JJ line: the 6550, 6CA7 and EL844. The JJ 6550 is musical in every sense of the word and will improve the sound of your high powered bass and guitar amps with punch and clarity. The 6CA7 is a large bottle version of the classic EL34 that gives a bigger and warmer sound then any EL34 in the market today. It is a substitute for all amps that use EL34’s. The last addition to the line has been designed by JJ from scratch, an EL844. A substitute for any EL84, the EL844 has lower gain and a more balanced, dynamic sound.

The complete line of JJ tubes, along with thousands of other parts, is available wholesale for dealers, repair shops, and manufacturers from CE Distribution (www.cedist.com).