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Tube Amp Maintenance & Handling Tips

March 28, 2014 by · 1 Comment 

Guitar Amplifier ExplodesTo get the most out of your tube amp, which can be a hefty investment, here are some easy to follow handling and maintenance tips. There’s no arguing that tube amps still set the standard for tone – modern solid state and digital amps merely emulate the tube amps of the past. The unfortunate drawback however, is that tube amps are more expensive and fragile compared to solid state alternatives. Repairs and maintenance add to the already high costs of tube amps. Thankfully, some of these extra expenses can be avoided with proper care and maintenance. Read more

Field Guide to Guitar Amplifier Tubes – Everything You Need To Know

March 27, 2013 by · 3 Comments 

Guitar amplifier valve tubesWhat is a vacuum tube? — If the link between a light bulb and a raging Marshall stack doesn’t seem obvious, take a glance at those vacuum tubes glowing inside your amp. And then pay your respects to Thomas Edison, the Wizard of Menlo Park, New Jersey.

Edison produced an incandescent light bulb in the late 1870s by feeding an electric current to a filament until it burned bright red. He then sheathed the filament in a glass bulb and extracted the oxygen, thus creating a vacuum to prevent the filament from burning up.

During further experiments in 1883, Edison detected electrons flowing from the filament, through the vacuum and over to a metal plate he had placed inside the bulb. This discovery, named the Edison Effect, remained a minor phenomenon until another scientist, John Fleming, found that these electrons could be used to detect radio waves and convert them into electricity. Fleming added a second metal element to the bulb, thereby creating the first diode, or—to use a name more familiar to guitarists—rectifier. In 1906, inventor Lee de Forest added a third element to the bulb and found that the resulting triode could be used as both a switch and an amplifier. Today, the vacuum tube has evolved to include a complex assembly of plates, grids and heating elements that produce a more efficient part. Read more

Electric Guitar Pickups 101

December 1, 2011 by · Leave a Comment 

How electric guitar pickups workAn electric guitar’s sound is deeply influenced by its pickups. For a musician who has little or no knowledge of electronics the matter can often seem to be incredibly complicated. When it comes to electronics in general, pickups are quite easy to wrap your brain around. This article will explain the connection between the electronics and sound. Read more

How to Listen to and Care for Your Amp When It’s Sick

November 3, 2011 by · Leave a Comment 

Fender Vibro KingGuitar amps are, no matter how you cut it, black boxes. You may think you have control over them, but when it gets down to it, you can’t really see inside the black box, and even if you could, there aren’t any moving parts. Electricity is largely non-mechanical. Or in other words, magic. And when the magic stops, most people think all they can do is resort to prayer. Or an amp technician.

When your amp isn’t performing up to snuff, there’s still a lot you can do without having an EE degree, or even knowing how to operate test equipment. Here are some holistic approaches—and solutions—you can try yourself, as I did when my tube amp went on the fritz. Warning: Some of the following procedures involve messing around with the components of the amp, so be careful. Electricity can kill you. Proceed at your own risk. Read more

The Audible Frequency Range and Describing Tone

October 26, 2011 by · Leave a Comment 

Guitar ToneAs guitarists, most of us sooner or later find ourselves in pursuit of tone.  A talented guitarist can find a way to make anything sound good, but there should be no doubt that our equipment and the tone it provides can inspire and help fuel our creativity.  In pursuit of tonal inspiration, we need to develop a vocabulary to help us find what we’re looking for in our sound. 

The Audible Frequency Range

Most guitarists start out by learning the names of the musical notes corresponding to a particular string and fret number, but they are not initially aware that these notes also correspond to the fundamental frequency of the vibrating string.  For example, the sixth string played at the 5th fret (low A) in standard tuning has a fundamental frequency of 110 Hz.  Any doubling or halving of a frequency is an octave, so the next octave up from 110 Hz would be 220 Hz.  In order to develop a vocabulary for tone, we have to think in terms of frequencies as opposed to musical notes. Read more

Guitar Technology: Smash or Trash?

October 11, 2011 by · Leave a Comment 

Guitar TechnologyGuitarists have always had an up-and-down relationship with the concept of high technology. Maybe it is because of the elemental nature of our instrument, especially in its acoustic form, or maybe it is due to the raw and often emotional approach taken by many of the creators and ongoing purveyors of rock, blues, country, and other styles of music. Whatever the case, there will always be a large part of the guitar community that likes to keep things simple and gets it done with a fairly traditional guitar, a tube amp, and a few pedals. Read more

Basic Electric Guitar Circuits: Potentiometers and Tone Capacitors

September 15, 2011 by · Leave a Comment 

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.

Potentiometers and Tone Capacitors

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.
Potentiometers and Tone Capacitors

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).

Potentiometers and Tone Capacitors

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).

Potentiometers and Tone Capacitors

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.
Potentiometers and Tone Capacitors

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.


Basic Electric Guitar Circuits | Guitar Pickups 101

August 26, 2011 by · Leave a Comment 

Guitar Pickups Single-Coil vs HumbuckerPassive (i.e. battery-free) electric guitar circuits are relatively simple and the possibilities for customization are endless.  A basic understanding of pickups, potentiometers, capacitors and switches is all you need to get creative and take more control of your instrument’s voice on an electronic level. Read more

Beer Can Guitar Amp

June 28, 2011 by · Leave a Comment 

Beer Can Guitar AmplifierThe first step in building your very own beer can amplifier will be to get an empty can of the beer of your choice. After you have your empty can you will need to remove the bottom of the can. With a little bit of patience and a razor knife, this can be done quite easily. Read more

JackTight Review: Guitar Repair Tool

April 14, 2011 by · Leave a Comment 

Guitar input jack repair toolEver had an issue with the input jack on your guitar, bass, or amplifier coming loose? Sometimes, it can be a real pain. Fear no more! The all-new JackTight is here to save the day! Read more

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