thumb|upright=1.2|A collection of guitar amplifier heads, cabinets, and combos.
A guitar amplifier (or amp) is an instrument amplifier designed for use with an electric guitar, bass guitar, or acoustic-electric guitar. An amplifier receives an electric guitar's pickup-generated signal and shapes that signal's tone through a preamplifier and amplifies it through a power amplifier; it is then projected through loudspeakers. When the preamp and power amp circuits are combined with one or more loudspeakers in a single enclosure, it is known as a combo. When the preamp and power amp are housed together for use with an external speaker cabinet, 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 usually paired with large external cabinets. The components chosen for an amp's circuit have a significant impact on its tonal character, which can be further sculpted 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 guitar 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 electronics and digital signal processing are also used. Guitarists often use external effects pedals to alter their tone before the signal reaches the amplifier.
History
1920s–1940s: Early models
thumb|right|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. 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
thumb|right|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.
1970s–1980s: Master volume amplifiers
thumb|right|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. 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. Throughout the 1980s, Mesa/Boogie continued revising its Mark series, pioneering new features with their Mark II 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.
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. The Kemper Profiler followed in 2011, pioneering "profiling", also known as "capturing", a technology that allows users to create and use digital versions of their own physical gear. The success of digital modeling in an otherwise analog landscape has led to a long-running debate over the merits of tube amplifiers compared to modelers. Tube amp players often appreciate the nostalgia of the technology and argue analog amps and cabinets have a "soul" that cannot be replicated; modeler users value their portability, consistency, and reliability. Boss debuted its Katana line of analog-digital hybrid amplifiers in 2016, and it became the industry's bestselling amp line.
As digital signal processing technology has progressed, modelers have become more compact and inexpensive, with budget offerings from brands like Mooer, NUX, and Joyo. Many brands now incorporate AI and machine learning into their products and design process.
Structure
Format
Fundamentally, guitar amplifiers function by taking the signal generated by an electric guitar's pickup(s) and feed it through an electronic circuit in which the tone is shaped and amplified before being projected through one or more loudspeakers. An amplifier in which this entire process happens inside a single enclosure is referred to as a combo. When the tone-shaping and amplification occur inside a dedicated unit that requires a separate speaker cabinet, it is known as a head. The amount of amplification applied to the incoming signal, known as gain, affects the amp's volume but to a greater extent affects its preamp distortion. The power amp then receives the processed signal and amplifies it to the player's desired output level via the loudspeakers. Wattage is used as a measurement of how much power an amp can produce. For many players though, the output level setting is chosen not purely for reasons of volume, but also for the perception that tube amps often sound their best at specific volume settings that strike a balance between driving the preamp and power amp while preserving the dynamics of a guitarist's playing. Preamp gain is often preferred for its added compression and harmonic overtones. High-gain abilities are now considered a definitive quality of modern amplifiers. With guitar amps, most open back amp cabinets are not fully open; part of the back is instead enclosed with panels. Combo amp cabinets and standalone speaker cabinets are typically made of plywood. Some are made of MDF or particle board—especially in low-budget models.
Types
Vacuum tube
thumb|Four Electro-Harmonix [[KT88 power tubes]]
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 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 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 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.
Pedal
thumb|A [[Universal Audio (company)|Universal Audio Dream '65 digital modeler preamp pedal.]]
A pedal amplifier is a compact amplifier placed at a player's feet, often as part of—or used in tandem with—a pedalboard. Pedal amps are effectively amp heads in pedal format, containing preamp and power amp circuits but not speakers. Pedal amps became popular in the 2020s as amplification technology continued to shrink in size, since they proved ideal options for a small, portable rig or for use as an emergency backup for a conventional amplifier.