Guitar amplifier

For amplifiers for bass guitar, see Bass amplifier.

Mesa/Boogie Mark IV, a guitar combo amplifier

A guitar amplifier (or amp) is an instrument amplifier designed for use with an electric guitar, bass guitar, or acoustic guitar. When a guitar amplifier's preamplifier and power amplifier circuits are combined with a speaker cabinet and one or more loudspeakers, it is known as a combo. When the circuits are housed in a dedicated enclosure, it is known as a head. There is a wide range of sizes and power ratings for guitar amplifiers, from small, low-wattage practice combos to heavy, 100-watt (or greater) heads that are often paired with large external cabinets. In addition to amplifying the guitar's signal, amps typically modify its tone by emphasizing or de-emphasizing certain frequencies using equalizer controls and through producing distortion (also known as overdrive). Reverb is a common built-in effect.

Commercially available amplifiers were first released in 1928 but did not become widely used until the introduction of mass-produced solid-body electric guitars and basses in the 1950s. Notable brands include Fender, Marshall, Vox, and Mesa/Boogie. For players, their choice of amp and the settings they use are a key part of their tone or sound. Historically, guitar amplifiers have been designed around vacuum tubes, but solid-state technology and digital signal processing have also been used. Guitarists often use external effects pedals to alter their tone before the signal reaches the amplifier.

History

1920s–1940s: Early models

Fender Deluxe 1953

High-powered audio amplifiers were first developed for use as public address systems and in movie theaters. While initially bulky and expensive, technological developments in the 1920s allowed for smaller, portable models that became popular among musicians playing a variety of instruments, especially lap steel guitars. The first commercially available guitar amplifiers were released in 1928 by the companies Stromberg-Voisinet and Vega, although at the time there was little difference between these early amplifiers and portable PA systems. Models released through the mid-1930s typically used a thin, wooden cabinet, a metal chassis with no control panel, a single volume control, and one or two inputs. Power amp sections typically used output transformers built into their field coil speakers and generated under 10 watts of power, while their speakers were small, usually under 10 inches. Electro String Instruments released its first amplifier in 1932. Their design—using a small, rectangular wooden box with a cutout for a speaker and a handle on top—would serve as a template for other, pre-WWII models. An early pioneer of electric guitar, Alvino Rey used his Electro amp on hundreds of gigs and recordings at the time. In 1933, Dobro released its own amp model to compete with Electro's, whose amp would be updated with metal corners and a different grill cloth and released in 1934 under the Rickenbacker label. In 1935, Electro/Rickenbacher sold more amps and electric guitars than all the amps and electrified or electric guitars that had been made from 1928 through the end of 1934. Gibson also developed prototype amps around this time, but never released them.

In the mid-1940s, K&F Manufacturing released their first amplifiers, low-wattage combos with steel casings and no covering. The K&F partnership was short-lived, however, with co-founder Leo Fender starting his own namesake brand, Fender, to continue producing amplifiers, as well as electric guitars and basses. Fender's first amps had wooden enclosures and were characterized by wide control panels that made them resemble television sets of the time. Like pickup models of that era, these early amplifiers were not loud enough for use on stage: when players turned their amps up all the way, the signal clipped, losing high and low frequencies but gaining compression, harmonics, and a "musical type of distortion". While an unintended technical shortcoming, players embraced it and amp distortion would go on to become closely associated with the electric guitar. One such amp known to distort at high volumes, the Fender Deluxe of 1948, became regarded as one of the earliest amp designs to achieve iconic status. Like other Fender amp models in the 1940s and 1950s, the Deluxe came in a Tweed-covered cabinet.

1950s–1960s: Rock and roll

A Vox AC30

In 1950, Fender debuted its first mass-produced, solid-body electric guitars, the Esquire and Telecaster (then known as the Broadcaster). Using a solid-body design eliminated the unwanted feedback caused by pickups on acoustic jazz guitars when they were amplified. Fender released the Twin in 1950 and followed in 1952 with the bass-oriented Bassman. While the Twin excelled at clean tones, the Bassman's distortion at loud volumes became popular among both bassists and guitarists. The Bassman later served as inspiration for Marshall's debut amp model, the JTM45. Vox released its AC30 combo in 1958 and then updated the amp in 1960 as a three-channel model that became famous for its jangly tone and widespread use during the British Invasion. In response to players like Pete Townshend of The Who who wanted more power, Marshall released the 100-watt 1959 Super Lead in 1965. The JTM45 and Super Lead were heads, which combined the preamp and power amp in a dedicated enclosure that was paired with an external speaker cabinet. Pairing the Super Lead with one or more 4x12 cabinets became known as the "Marshall stack". The combination has been used by many notable guitarists of the era and played a major role in shaping the sound of rock and roll.

Throughout the 1960s, Fender continued updating the circuits and cosmetics of their amplifiers. Fender's Twin Reverb of the mid-1960s became a standard amp model for "clean", undistorted tones. Hiwatt's DR103 was similarly highly regarded as a loud, clean amplifier and has frequently been associated with David Gilmour. In 1965, Peavey released its first amp models, which used transistors rather than the standard vacuum tubes, which had been replaced by solid-state technology in most industries following WWII. By the 1970s, Fender, Vox, Marshall, and others had all released transistor-based amplifiers, with the most successful being the Roland Jazz Chorus, which became a popular alternative to the Fender Twin for clean tones.

1970s–1980s: Master volume amplifiers

JCM 800 head and 4x12 cabinet.

As rock music evolved, so too did the need among guitarists for greater amounts of distortion. Amps like Marshall's Super Lead could produce distortion but did so by a player pushing the power section into saturation by playing it at high volume levels that were not always practical. Many players turned to effects pedals like the Maestro Fuzz-Tone and Arbiter Fuzz Face as a solution, the former notably being used by Keith Richards on the Rolling Stones' hit "(I Can't Get No) Satisfaction".

By the early 1970s, it had become popular for technicians to "hot rod" Fender amplifiers to achieve more distortion through methods like adding a "master volume" circuit, which used two volume controls, one to control how much the preamp tubes distorted and another to control overall output. One such technician was Mesa/Boogie founder Randall Smith, whose modified Fenders evolved into the brand's Mark series, which used a series of variable gain stages that "cascaded" into each other to create more distortion than any previous amplifier could. In doing so, Smith pioneered "high-gain" amps. Howard Dumble used a similar cascading gain design with his Overdrive Special, which gave players a foot-switchable "lead" mode with extra gain stages.

While early Mesas and Dumbles were popular among guitarists like Carlos Santana, the heavier guitar tones of Black Sabbath's Tony Iommi using a Laney Supergroup amplifier and a modified Dallas Rangemaster treble booster inspired players in the growing heavy metal genre to pursue more aggressive tones. Marshall looked to meet this demand with the 1981 release of the JCM800 2203, which was initially a close copy of the brand's first master volume-equipped amp, the JMP 2203, released in 1975, but underwent several circuit changes in the next few years. Throughout the 1980s, Mesa/Boogie continued revising its Mark series, pioneering new features with their MKII iterations like channel-switching and effects loops. Smith and his company's most significant revision produced the Mark IIC+ variant, which had a tighter, more aggressive tone that was popularized by Metallica. The JCM800 and Mark series marked a transition from "vintage"-style amplifiers to modern designs.

Technicians in the 1980s continued pushing modded amplifiers into higher-gain territory, and some became successful enough to launch their own companies, including Soldano, Bogner, and Rivera. Soldano's SLO-100 helped establish the high-end, high-gain amplifier market and inspired later German brands like ENGL, Hughes & Kettner, and Diezel. Toward the end of the decade, many professional guitarists adopted complex rackmount setups, often consisting of multiple preamps, power amps, and studio-grade effects, all operated with a custom pedal controller. Popular rackmount preamps included the A/DA MP-1, Mesa/Boogie Triaxis, Marshall JMP-1, and Soldano X88-R.

1990s–present: Boutique brands and digital modeling

While rackmount setups were widely used by pros, they were expensive, and by the time affordable rackmount options came to the market in the early 1990s grunge had inspired a return to standalone amplifiers, vintage-style effects pedals, and a less processed sound. The 1990s also experienced a new wave of boutique amplifier builders. The old Fender and Marshall amps previous techs had modded were by then too valuable to alter, so many instead founded companies that built new amps inspired by vintage designs. A notable example was Matchless and their Vox AC30-inspired DC-30 combo. Other companies continued to push the boundaries of high-gain, with Mesa/Boogie's Dual Rectifier becoming the decade's quintessential high-gain amplifier. Peavey's 5150, co-designed with Eddie Van Halen, became another frequently-used amplifier among metal guitarists.

An Axe-FX Ultra

Modeling technology advanced significantly during this time. Tech 21 had released its analog SansAmp amplifier modeler in 1989 but it became popular in the 1990s as a direct recording solution. Line 6 released its first digital modeling amplifier, the AxSys, in 1996, followed by the bean-shaped desktop POD processor two years later. Early modelers were regarded as home practice tools until the arrival of Fractal Audio's Axe-FX, the first modeler considered sufficient for professional use. The Kemper Profiler followed in 2006, pioneering profiling, also known as capturing, a technology that allows users to create and use digital versions of their own physical gear. Neural DSP established itself through audio plug-ins that create computer-based virtual amplifier and effects modelling suites. Neural released their first piece of hardware, the Quad Cortex, in 2020.

Many tube amp manufacturers have responded by implementing digital technology into their products, especially impulse responses (IRs), which are digital emulations of speaker cabinets and microphones. IRs allow for easier home recording, letting players skip the use of physical cabinet setups. Alongside the development of modelers, smaller "lunchbox" amplifiers—amp heads that use metal cases and a low-output power stage—have become popular following the success of the 15-watt Orange Tiny Terror, released in 2006. Boss debuted its Katana line of analog-digital hybrid amplifiers in 2016, and it became the industry's bestselling amp line.

Structure

A Fender Bassman amp head with a 15" speaker cabinet

Typically, guitar amplifiers have two amplifying circuit stages, and frequently have tone-shaping electric circuits, which usually include at least bass and treble controls, which function similarly to the equivalent controls on a home hi-fi system. More expensive amplifiers typically have more controls for other frequency ranges, such as one or two midrange controls and a presence control for high frequencies. Some guitar amplifiers have a graphic equalizer, which uses vertical faders to control multiple frequency bands. Some more expensive bass amps have a parametric equalizer, which enables precise control of tone.

The first amplifier stage is a preamplifier. It amplifies the audio signal to a level that can drive the power stage. The preamplifier also changes the tone of the signal; high preamp settings add overdrive. The power amplifier produces a high current signal to drive a loudspeaker and produce sound.

Various types of tone stages may affect the guitar signal:

• Settings on the guitar itself (passive tone controls, active equalizer circuits in built-in preamps, pickup selector switch position, etc.)
• Devices between the guitar and the preamp stage, such as a wah-wah pedal or other effects units, such as chorus or reverb.
• Between the preamp and power stages (an effects loop or some dedicated amplifier tone circuits)
• Between multiple stacked preamp stages (also called gain stages)
• In feedback loops from a post-preamp signal to an earlier pre-preamp signal (as in the case of presence modifier circuits)

Tone stages may also provide electronic effects—such as equalization, compression, distortion, chorus, or reverb. Amplifiers may use vacuum tubes (called valves in Britain), solid-state (transistor) devices, or both.

The two common guitar amplifier configurations are a combination ("combo") amplifier that includes an amplifier and one or more speakers in a single cabinet, and a standalone amplifier (often called a head or amp head), which passes the amplified signal via a speaker cable to one or more external speaker cabinets. A wide range of speaker configurations are available in guitar cabinets—from cabinets with a single speaker (e.g., 1×10" or 1×12") or multiple speakers (e.g., 2×10", 4×10" or 8x10").

Guitar amplifiers vary widely in price and quality. Many music equipment companies import small, low-powered practice amplifiers for students and beginners that sell for less than $50. Other companies produce expensive custom-made amplifiers for professional musicians, which can cost hundreds or even thousands of dollars (USD). Most combo amplifiers have a carrying handle, and many combo amplifiers and cabinets have metal or plastic-reinforced corners to protect the amp during transportation.

Control knobs and buttons are typically on the front of the cabinet or chassis, though in some cases, the knobs are on a recessed panel at the back of the top of the amplifier. The most basic amps only have a few knobs, which typically control volume, bass, and treble. More expensive amps may have several knobs that control pre-amp volume (or gain), distortion or overdrive, volume, bass, mid and treble, and reverb. Some older amps (and their re-issued versions) have a knob that controls a vibrato or tremolo effect. The 1/4" input jack is typically mounted on the front of the amplifier. In the simplest, least expensive amplifiers, this 1/4" jack is the only jack on the amplifier.

The patch bay at the rear panel of this Line 6 Flextone guitar amp provides several additional inputs and outputs, including stereo XLR DI unit outputs.

More expensive amplifiers may have a patch bay for multiple inputs and outputs, such as a pre-amp out (for sending to another guitar amplifier), a second low gain input, to use with active basses, an in jack to create an effects loop (when used with the pre-amp out jack), an external speaker output (for powering an additional speaker cabinet), and stereo RCA jacks or a 1/8" jack, for connecting a CD player or MP3 player so that a player can practice along with recorded music. Some amps have a 1/4" jack for connecting a pedal to turn the amp's onboard overdrive and reverb on and off or to switch between channels. Some amps have an XLR jack for a microphone, either for the guitar amp to be used for singing (in effect as a mini-PA system), or, for acoustic guitar, to mix a mic signal with a pickup signal.

The vast majority of guitar amps can only be powered by AC mains power (plugging into a wall outlet); however, a small number of practice amps are designed for buskers also have battery power so they can be used for street performances.

Types

Kustom 200 bass amp – amp head and speakers, 100 watts RMS, two channels, two 15" speakers, 1971

A combo amp contains the amplifier and one or more speakers in a single cabinet. In a head and speaker cabinet configuration, the amplifier and speaker each have their own cabinet. The amplifier (head) may drive one or more speaker cabinets.

In the 1920s, guitarists played through public address amplifiers, but by the 1940s this was uncommon.

Besides instrument inputs and speaker outputs (typically via 1/4" jacks), an amp may have other inputs and outputs. These can include an auxiliary input jack (sometimes with its own level control, for a drum machine), send and return jacks to create an effects loop, a line out jack, and an extension speaker jack. Practice amps sometimes have a 1/4" headphone jack, or stereo RCA or mini jacks for connecting a CD player, portable media player or other sound sources. Some guitar amps have an XLR input so that a microphone can be plugged in for singing. Guitar amps that include a mic input are in effect small, portable PA systems. Some amps, typically bass amps, have an XLR connector to provide a balanced output from the preamp section to a PA system or recording input.

Instrument amplifiers are available in a wide range of price, quality, and performance levels. Some are designed for beginners, such as small, low-wattage practice amps, which typically have a single 8" speaker and about 10 watts, or smaller combo amps with relatively low wattage (15 to 20 watts) and a single 10" speaker. Mid- to large-size combo amps with 30 to 50 watts and one 12" speaker or four 10" speakers are best for high-volume situations, such as band rehearsals and onstage performances. For large venues, such as outdoor music festivals, guitarists may use one or more 100 watts (or several hundred watts) heads with one or more 8x10” cabinets.

Vacuum tube

The glow from four "Electro Harmonix KT88" brand power tubes lights up the inside of a Traynor YBA-200 bass guitar amplifier

Main article: Valve amplifier

Vacuum tubes (called valves in British English) were by far the dominant active electronic components in most instrument amplifier applications until the 1970s when solid-state semiconductors (transistors) started taking over. Transistor amplifiers are less expensive to build and maintain, reduce the weight and heat of an amplifier, and tend to be more reliable and more shock-resistant. Tubes are fragile and they must be replaced and maintained periodically. As well, serious problems with the tubes can render an amplifier inoperable until the issue is resolved.

While tube-based circuitry is technologically outdated, tube amps remain popular since many guitarists prefer their sound. Tube enthusiasts believe that tube amps produce a warmer sound and a more natural "overdrive" sound.

Solid-state

Most inexpensive and mid-priced guitar amplifiers are based on transistor or semiconductor (solid-state) circuits, which are cheaper to produce and more reliable, and usually much lighter than tube amplifiers. Solid-state amps are less fragile than tube amps.

High-end solid-state amplifiers are less common, since many professional guitarists favor vacuum tubes. Some jazz guitarists favor the cleaner sound of solid-state amplifiers. Only a few solid-state amps have enduring attraction, such as the Roland Jazz Chorus. Solid-state amplifiers vary in output power, functionality, size, price, and sound quality in a wide range, from practice amplifiers to combos suitable for gigging to professional models intended for session musicians who do studio recording work.

Hybrid

A hybrid amplifier involves one of two combinations of tube and solid-state amplification. It may have a tube power amp fed by a solid-state pre-amp circuit, as in most of the original MusicMan amplifiers.

Alternatively, a tube preamplifier can feed a solid-state output stage, as in models from Kustom, Hartke, SWR, and Vox. This approach dispenses with the need for an output transformer and easily achieves modern power levels.

Modeling

A modeling amplifier, shown from above. Note the various amplifier and speaker emulations selectable via the rotary knob on the left.

Microprocessor technology allows the use of digital onboard effects in guitar amps to create numerous different sounds and tones that simulate the sound of a range of tube amplifiers and different sized speaker cabinets, all using the same amplifier and speaker. These are known as modeling amplifiers, and can be programmed with simulated characteristic tones of different existing amplifier models (and speaker cabinets—even microphone type or placement), or dialed in to the user's taste. Many amps of this type are also programmable by way of USB connection to a home computer or laptop. Line 6 is generally credited with bringing modeling amplification to the market. Modeling amplifiers and stompbox pedals, rackmount units, and software that models specific amplifiers, speakers cabinets, and microphones can provide a large number of sounds and tones. Players can get a reasonable facsimile of the sound of tube amplifiers, vintage combo amplifiers, and huge 8x10” speaker stacks without bringing all that heavy equipment to the studio or stage.

The use of full range, flat response (FRFR) amplification systems by electric guitarists has received an extra impetus from modeling amplifiers. Before widespread availability of modeling, guitarists did not commonly plug electric guitars straight into PA systems or powered speakers because most genres relied on the tonal coloration of a regular guitar amplifier setup—from the preamplifier, equalization filters, power amp, guitar speakers, and cabinet design. The FRFR approach assumes the tone is shaped by sound processors in the signal chain before the amplifier and speaker stage, so it strives to not add further coloration or dedicated combo-style amplifiers with a broad frequency range. Such processors can be traditional guitar effects, a modeling amplifier (without power amplifier), or a computer running tone-shaping software. Using a modeling amp or a multi-effects pedal used with line level output, a guitarist can plug in the guitar into a flat response mic input or into a keyboard amplifier.

Acoustic

Acoustic amplifiers are intended for acoustic guitars and other acoustic instruments, especially for the way these instruments are used in relatively quiet genres such as folk and bluegrass. They are similar to keyboard amplifiers, in that they have a relatively flat frequency response with minimal coloration. To produce this relatively clean sound, these amplifiers often have powerful amplifiers (providing up to 800 watts RMS), to provide additional headroom and prevent unwanted distortion. Since an 800-watt amplifier built with standard Class AB technology is heavy, some acoustic amplifier manufacturers use lightweight Class D amplifiers, which are also called switching amplifiers.

Acoustic amplifiers produce an uncolored, acoustic sound when used with acoustic instruments with built-in transducers, pickups or microphones. The amplifiers often come with a simple mixer, so that the signals from a pickup and a condenser microphone can be blended. Since the early 2000s, it has become increasingly common for acoustic amplifiers to provide a range of digital effects, such as reverb and compression. As well, these amplifiers often contain feedback-suppressing devices, such as notch filters or parametric equalizers.

Stacks

A 3 × 6 stack of mock Marshall guitar cabinets for Jeff Hanneman of Slayer

An amplifier stack consists of an amplifier head atop a speaker cabinet—a head on top of one cabinet is commonly called a half stack, a head atop two cabinets a full-stack. The cabinet that the head sits on often has an angled top in front, while the lower cabinet of a full stack has a straight front. The first version of the Marshall stack was an amp head on an 8×12 cabinet, meaning a single speaker cabinet containing eight 12" guitar speakers. After six of these cabinets were made, the cabinet arrangement was changed to an amp head on two 4×12 (four 12" speakers) cabinets to make the cabinets more transportable. Some touring metal and rock bands have used a large array of guitar speaker cabinets for their impressive appearance. Some of these arrangements include only the fronts of speaker cabinets mounted on a large frame.

There are many varieties of speaker combinations used in guitar speaker cabinets, including one 12" speaker, one 15" speaker (this is more common for bass amplifiers than for electric guitar cabinets), two 10" speakers, four 10" speakers, four 12" speakers, or eight 10" speakers. Less commonly, guitar cabinets may contain different sizes of speakers in the same cabinet. Cabinets with eight 10" speakers are large and heavy, and they are often equipped with wheels and a towel bar-style handle for transport. Some cabinets use mixed speaker types, such as one 15" speaker and two 10" speakers.

Cabinet design

Combo guitar amplifier cabinets and guitar speaker cabinets use several different designs, including the open back cabinet, the closed back cabinet (a sealed box), and, less commonly, bass reflex designs, which use a closed back with a vent or port cut into the cabinet. With guitar amps, most open back amp cabinets are not fully open; part of the back is enclosed with panels. Combo guitar amp cabinets and standalone speaker cabinets are often made of plywood. Some are made of MDF or particle board—especially in low-budget models. Cabinet size and depth, material types, assembly methods, type and thickness of the baffle material (the wood panel that holds the speaker), and the way the baffle attaches to the cabinet all affect tone.

When two or more speakers are used in the same cabinet, or when two cabinets are used together, the speakers can be wired in parallel or in series, or in a combination of the two (e.g., two 2x10" cabinets, with the two speakers wired in series, can be connected together in parallel). Whether speakers are wired in parallel or in series affects the impedance of the system. Two 8 ohm speakers wired in parallel have 4-ohm impedance. Guitarists who connect multiple cabinets to an amplifier must consider the amp's minimum impedance. Parallel vs. series also affects tone and sound. Speakers wired in parallel slightly dampen[s] and restrain[s] them, giving what some describe as tighter response and smoother breakup. Some describe speakers wired in series (usually no more than two) as sounding "...looser, giving a slightly more raw, open and edgy sound."

Distortion, power, and volume

Power output

The relationship between power output in watts and perceived volume is not immediately obvious. The human ear perceives a 5-watt amplifier as half as loud as a 50-watt amplifier (a tenfold increase in power), and a half-watt amplifier is a quarter as loud as a 50-watt amp. Due to this logarithmic scale, doubling the output power of an amplifier results in a just noticeable increase in volume, so a 100-watt amplifier is only just noticeably louder than a 50-watt amplifier. Such generalizations are also subject to the human ear's tendency to behave as a natural compressor at high volumes.

Power attenuation can be used with either low-power or high-power amplifiers, resulting in variable-power amplifiers. A high-power amplifier with power attenuation can produce power-tube distortion through a range of listening volumes but with a decrease in high-power distortion. Other technologies, such as dual rectifiers and the sag circuit—which should not be confused with attenuation—allow high-power amplifiers to produce low power volume while preserving high power distortion.

Speaker efficiency is also a major factor affecting a tube amplifier's maximum volume.

For bass instruments, higher-power amplifiers are needed to reproduce low-frequency sounds. While an electric guitarist would be able to play at a small club with a 50-watt amplifier, a bass player performing in the same venue would probably need an amplifier with 200 or more watts.

Distortion and volume

Marshall is a popular amplifier manufacturer for metal and hard rock. Pictured is the MG15DFX guitar amplifier.

Distortion is a feature available on many guitar amplifiers that is not typically found on keyboard or bass guitar amplifiers. Tube guitar amplifiers can produce distortion through pre-distortion equalization, preamp tube distortion, post-distortion EQ, power-tube distortion, tube rectifier compression, output transformer distortion, guitar speaker distortion, and guitar speaker and cabinet frequency response. Because many factors beyond preamp distortion contribute to a particular guitarist's sound, recording engineers and PA system techs typically put a microphone in front of the guitar speaker, rather than only use the guitar amp's pre-amp out signal. A sound engineer or music producer may send the DI out signal from the pickups to a separate track at the same time, so they can re-amp the signal later. In contrast, it is fairly common to use a DI box with electric bass.

Distortion sound or "texture" from guitar amplifiers is further shaped or processed through the frequency response and distortion factors in the microphones (their response, placement, and multi-microphone comb filtering effects), microphone preamps, mixer channel equalization, and compression. Additionally, the basic sound produced by the guitar amplifier can be changed and shaped by adding distortion and/or equalization effect pedals before the amp's input jack, in the effects loop just before the tube power amp, or after the power tubes.

Power-tube distortion

Power-tube distortion is required for amp sounds in some genres. In a standard master-volume guitar amp, as the amp's final or master volume is increased beyond the full power of the amplifier, power-tube distortion is produced. The "power soak" approach places the attenuation between the power tubes and the guitar speaker. In the re-amped or "dummy load" approach, the tube power amp drives a mostly resistive dummy load while an additional low power amp drives the guitar speaker. In the isolation box approach, the guitar amplifier is used with a guitar speaker in a separate cabinet. A soundproofed isolation cabinet, isolation box, isolation booth, or isolation room can be used.

Volume controls

A Marshall JCM 900's knobs for equalization, gain, reverb and volume

A variety of labels are used for level attenuation potentiometers (knobs) in a guitar amplifier and other guitar equipment. Electric guitars and basses have a volume control on the instrument that attenuates the signal from selected pickups. There may be two volume controls on an electric guitar or bass, wired in parallel to mix the signal levels from the neck and bridge pickups. Rolling back the guitar's volume control also changes the pickup's equalization or frequency response, which can provide pre-distortion equalization.

The simplest guitar amplifiers, such as some vintage amps and modern practice amps, have only a single volume control. Most have two volume controls: a first volume control called "preamplifier" or "gain" and a master volume control. The preamp or gain control works differently on different guitar amp designs. On an amp designed for acoustic guitar, turning up the preamp knob pre-amplifies the signal—but even at its maximum setting, the preamp control is unlikely to produce much overdrive. However, with amps designed for electric guitarists playing blues, hard rock and heavy metal music, turning up the preamp or gain knob usually produces overdrive distortion. Some electric guitar amps have three controls in the volume section: pre-amplifier, distortion, and master control. Turning up the preamp and distortion knobs in varying combinations can create a range of overdrive tones, from a gentle, warm growling overdrive suitable for a traditional blues show or a rockabilly band to the extreme distortion used in hardcore punk and death metal. On some electric guitar amps, the "gain" knob is equivalent to the distortion control on a distortion pedal and similarly may have a side-effect of changing the proportion of bass and treble sent to the next stage.

A simple, inexpensive amplifier may have only two tone controls, a passive bass and treble control. In some better-quality amps, one or more midrange controls are provided. On the most expensive amps, there may be shelving equalizers for bass and treble, several mid-range controls (e.g., low mid, mid, and high mid), and a graphic equalizer or parametric equalizer. The amplifier's master volume control restricts the amount of signal permitted through to the driver stage and the power amplifier. When using a power attenuator with a tube amplifier, the master volume no longer acts as the master volume control. Instead, the power attenuator's attenuation control controls the power delivered to the speaker, and the amplifier's master volume control determines the amount of power-tube distortion. Power-supply-based power reduction is controlled by a knob on the tube power amp, variously labeled wattage, power, scale, power scale, or power dampening.

In popular culture

The original "up to eleven" knobs in the 1984 film This Is Spinal Tap

Volume control gradations are typically numbered from zero to ten. In the 1984 film This Is Spinal Tap, guitarist Nigel Tufnel demonstrates an amplifier whose volume knobs are marked from zero to eleven, believing that this numbering increases the highest volume of the amp. He explains, "It's one louder, isn't it?" This misunderstanding of the underlying operating principles led to the idiom "Up to eleven", also phrased as "These go to eleven". As a consequence of the film, real bands and musicians started buying equipment whose knobs went up to eleven or twelve.

See also

• Vintage musical equipment
• Tube sound
• Bass amplifier

Further reading

• Fliegler, Ritchie. The Complete Guide to Guitar and Amp Maintenance. Hal Leonard Corporation, 1994.
• Fliegler, Ritchie and Eiche, Jon F. Amps!: The Other Half of Rock 'n' Roll. Hal Leonard Corporation, 1993.
• Hunter, Dave. Amped: The Illustrated History of the World's Greatest Amplifiers. Voyageur Press, 2012.
• Pittman, Aspen. The Tube Amp Book. Backbeat, 2003.
• Tarquin, Brian. Guitar Amplifier Encyclopedia. Skyhorse Publishing, Inc., 2016.
• Weber, Gerald, "A Desktop Reference of Hip Vintage Guitar Amps", Hal Leonard Corporation, 1994. ISBN 0-9641060-0-0

External links

• Lenard Audio History of Guitar Amplifiers (non-profit educational website)
• Vintage Guitar magazine article on vintage amps