Guitar amplifier

A guitar combo amplifier

A guitar amplifier is an electronic amplifier designed for use with an electric or electronic musical instrument, such as an electric guitar.

History

The first electronic instrument amplifiers were designed for use with electric guitars. The earliest examples appeared in the early 1930s when the introduction of electrolytic capacitors and rectifier tubes allowed for the production of economical built-in power supplies that could be plugged into wall sockets. Consequently portable vacuum tube amplification equipment was no longer dependent on heavy multiple battery packs for power. While guitar amplifiers from the beginning were used to amplify acoustic guitar, electronic amplification of guitar was first widely popularized by the 1930s and 1940s craze for Hawaiian music, which extensively employed the amplified lap steel Hawaiian guitar.

Tone controls on early guitar amplifiers were very simple and provided a great deal of treble boost but the controls, the loudspeakers used and the low power of the amplifiers (typically 15 watts or less prior to the mid-1950s) had poor high treble and bass response. Some better models also provided effects such as spring reverb and/or an electronic tremolo unit. Early Fender amps labeled tremolo as "vibrato" and labeled the vibrato arm of the Stratocaster guitar as a "tremolo bar"; see vibrato unit, electric guitar, and tremolo).

In the 1960s, guitarists experimented with distortion produced by deliberately overloading (or overdriving) their amplifiers. The Kinks guitarist Dave Davies produced early distortion effects by connecting the output of one amplifier into the input of another, an abuse that the designers could never have imagined. Later, most guitar amps were provided with preamplifier distortion controls, and "fuzz boxes" and other effects units were engineered to safely and reliably produce these sounds. Today, distortion has become an integral part of many styles of electric guitar playing.

Guitar amplifiers were at first used with bass guitars and electronic keyboards, but when broader-bandwidth sounds are needed, other instruments use a suitable full-range speaker system and different power level. Much more amplifier power is required to clearly reproduce low-frequency pitches produced by bass guitars and electronic keyboards, especially at high volumes. Reproducing low-frequency pitches also requires a woofer or subwoofer capable of handling low frequencies and a speaker cabinet that is designed for low-frequency reproduction. Such speaker cabinets need to be larger and more sturdily built than speaker cabinets for mid-range or high-frequency sounds.

Most common forms

Two combo amplifiers

A full bass stack

Guitar amplifiers are manufactured in two main forms. The combination (or combo) amplifier contains the amplifier head and guitar speakers in a single unit. The amplifier head or amp head contains the electronic circuitry constituting the preamp, built-in effects processing, and power amplifier. In the other form, the amplifier head is separate from the speakers, and joined to them by cables. The separate amplifier is called an amplifier head, and is commonly placed on top of one or more loudspeaker enclosures. A separate amplifier head placed atop a guitar speaker enclosure or guitar speaker cabinet forms an amplifier stack or amp stack.

Some amplifiers used with electric guitars are solid state, because they are easy to repair, lighter-weight, and less expensive. Many guitarists, particularly in the genres of blues and rock, prefer the sound of vacuum tube amplifiers despite their drawbacks such as heavy weight, the need to periodically replace tubes and need to re-bias the output tubes (every year or two with moderate use). Some companies design amplifiers that require no biasing as long as properly rated tubes are used. Some modern amplifiers use a mixture of tube and solid-state technologies. Since the advent of microprocessors and digital signal processing, modeling amps have been developed in the late 1990s, these can simulate the sounds of a variety of well-known tube amplifiers without always using vacuum tubes. Amplifiers with processors and software emulate the sound of a classic amps almost perfectly, but from the player's point of view the response of these amplifiers is not quite the same due to the digital element of modeling. In blind auditory tests tube amplifier fans may not be able to tell the difference, but most will always choose to play a tube amp because of its analog sensitivity.

Use with other instruments

Musicians often run sound-sources other than guitars through guitar amps.

For live performances, synthesizers such as drum machines or keyboards are often put through guitar amps to create a richer sound than using their direct-outs. String instruments and vocals are also put through guitar amps to add distortion effects. Some blues harp players also use guitar amps for their distortion. Additionally, recording engineers occasionally run pre-recorded parts through miked guitar amps for similar reasons.

Mixing guitar amp signals with other signals is also common. Chris Squire of Yes produced his classic bass guitar sound by playing through a guitar amplifier with its bass turned down, treble turned up, and volume turned up well into distortion; the miked guitar speaker signal was then mixed with a direct-inject (DI) signal, a technique that has been used for processing synth keyboards as well. A bass guitar can also be played through a bass amp in parallel with a distorted guitar amp by using a DI box, producing complex breakup yet well-defined lower frequencies.

Types of instrument amplifiers

A wide range of instrument amplifiers are now available, some general purpose and some designed for specific instruments and even for particular sounds. These include:

• "Traditional" guitar amplifiers, with a clean, warm sound, a sharp treble roll-off at 5 kHz or less and bass roll off at 60–100 Hz, and often built-in reverb and tremolo ("vibrato") units. These amplifiers, such as the Fender "Tweed"-style amps, are often used by traditional rock, blues, and country musicians.Traditional amps have more recently become popular with musicians in indie and alternative bands
• Hard rock-style guitar amplifiers, which often include a preamplification controls, tone filters, and distortion effects that provide the amplifier's characteristic tone. Users of these amplifiers use the amplifier's tone to add "drive", intensity, and "edge" to their guitar sound. Amplifiers of this type, such as Marshall amplifiers, are used in a range of genres, including hard rock, metal, and punk.
• Bass amplifiers, with extended bass response and tone controls optimized for bass guitars (or more rarely, for upright bass). Higher-end bass amplifiers sometimes include compressor or limiter features, which help to keep the amplifier from distorting at high volume levels, and an XLR DI output for patching the bass signal directly into a mixing board. Bass amplifiers are often provided with external metal heat sinks or fans to help keep the amplifier cool.
• Keyboard amplifiers, with very low distortion and extended, flat frequency response in both directions. Keyboard amplifiers often have a simple onboard mixer, so that keyboardists can control the tone and level of several keyboards.
• Acoustic amplifiers, similar in many ways to keyboard amplifiers but designed specifically to produce a "clean," transparent, "acoustic" sound when used with acoustic instruments with built-in transducer pickups and/or microphones. (Note that there was once also a brand of guitar and bass amplifier called Acoustic, still seen second-hand.)

In addition to a 1/4" input jack, some instrument amplifiers have an additional input jack for a microphone, which is easily identified because it will use a three-pin XLR connector. Phantom power is not often provided on general-use amps, restricting the choice of microphones for use with these inputs. However, for high-end acoustic amplifiers, phantom power is often provided, so that musicians can use condenser microphones.

Vacuum tube amplifiers

Rear view of a tube (valve) combo guitar amplifier. Visible are two glass output tubes, six smaller preamp tubes in their metal tube retainers, and both the power transformer and the output transformer.

Vacuum tubes (valves) were by far the dominant active electronic components in most instrument amplifier applications until the 1970s, when semiconductors (transistors) started taking over for performance and economy reasons, including heat and weight reduction, and improved reliability. High-end tube instrument amplifiers have survived as one of few exceptions, because many players like their sounds.

A simple tube amplifier circuit is quite straightforward. Typically, at least two triodes are used in the preamplifier section in order to obtain a signal level high enough to implement any tone controls and drive the following sections. Another two triodes split the phase of the signal and drive the final pentode or beam tetrode (also known as "kinkless tetrodes", hence the KTxx nomenclature) tubes of the push-pull power amplifier stage. The output tubes are often arranged in a push-pull connection to obtain maximum power. Some high power models use paralleled pairs of output tubes (4 or more in total) in push-pull. Except for the light negative feedback from the secondary end of the output transformer to the driver stage, most amplifying stages work in "raw" open-loop mode, some designs employ current feedback via unbypassed cathode resistors.

Since most tubes show "soft clipping" gain non-linearity, applying an input signal high enough to overdrive any stage tends to produce favorably natural distortion. Today, most vacuum tube amplifiers are based on the ECC83/12AX7/7025 (dual triode) tubes for the preamplifier and driver sections and the EL84/6BQ5 or EL34/6CA7/KT77 or 6L6/KT66 or 6V6 tubes for the power output section. Some use the KT88/6550 beam power tubes in the output stage. The differing codes for equivalent tubes generally reflect those used by the original European or U.S.A. based manufacturers. These tubes are now mainly manufactured in Russia, China and Eastern European countries. Some amplifiers, such as the Marshall Silver Jubilee, use solid state components in the preamp, most commonly diodes, to create distortion, a design feature known as diode clipping.

Tube instrument amplifiers are often equipped with lower grade transformers and simpler power regulation circuits than those of hi-fi amplifiers. They are usually not only for cost-saving reasons, but also are considered as a factor in sound creation. For example, a simple power regulation circuit's output tends to sag when there is a heavy load (that is, high output power) and vacuum tubes usually lose gain factors with lower power voltages; this results in somewhat compressed sound volume and it could be criticized as a "poor dynamic range" in case of hi-fi amplifiers, but it can be favorably accepted as "good compression" or "long sustain" of sounds on a guitar amplifier. Some tube guitar amplifiers use a rectifier tube specifically for this reason.

Some models have a "spring reverb" unit that simulates the reverberation of an echoic ambient. A reverb unit usually consists of one or more coil springs driven by the preamplifier section using a transducer driver similar to a loudspeaker at one end and an electro-magnetic pickup and preamplifier stage at the other end that picks up the long sustaining spring vibration, which is then mixed with the original signal.

Some guitar amplifiers have a tremolo control. An internal oscillator generates a low frequency continuous signal that modulates the input signal's amplitude, producing a tremolo effect.

Tube amps have the following disadvantages in comparison to solid-state amps:

• They are bulky and heavy, primarily due to the iron in power and output transformers. Solid-state amplifiers still require power transformers, but are usually direct-coupled and don't need output transformers.
• Glass tubes are fragile, and require more care and consideration when equipment is moved repeatedly.
• Tube performance can deteriorate slightly over time before eventual catastrophic failure. When tube vacuum is maintained at a high level, though, excellent performance and life is possible.
• They are prone to pick up mechanical noises (microphonic noise), although such electro-mechanical feedback from the loudspeaker to the tubes in combo amplifiers may contribute to sound creation.
• They require a warm-up period (up to a few minutes) to allow tube cathodes to operate without damage and so prolong tube life. This is of particular importance for amplifiers with solid state rectifiers.

Tube amps have the following advantages over solid-state amps:

• Compared to semiconductors, tubes have a very low "drift" (of specs) over a wide range of operating conditions, specifically high heat/high power. Semiconductors are very heat-sensitive by comparison and this fact usually leads to compromises in solid-state amplifier designs.
• When a tube fails, it is replaceable. While solid state devices are also replaceable, it's usually a much more involved process (i.e., having the amplifier tested by a professional, removing the faulty component, and replacing it). For working musicians this is usually a huge problem by comparison to looking in the back of a tube amp at the tubes and simply replacing the faulty tube. In addition, tubes can easily be removed and tested, while transistors cannot.
• A tube's amplification tends to be more linear than an equivalent solid state device , resulting in a more predictable response to transients (abrupt changes in audio level). This is especially the case when signal levels approach the point of clipping. In a tube-powered amplifier, the transition from linearity to non-linearity is less abrupt than in a solid state unit, resulting in a less grating form of distortion at the onset of clipping. For this reason, many professional guitarists prefer the sound of an all-tube amplifier.

Solid-state amplifiers

Most low-end guitar amplifiers currently produced are based on semiconductor (solid state) circuits, and some designs incorporate tubes in the preamp stage for their subjectively warmer overdrive sound. Tubes create warm overdrive sounds because instead of cutting the peaked signal off, they more or less pull the peaked audio information back (like natural compression) which creates a fuzzy overdrive sound. While this is a desirable attribute in many cases, the tube's characteristic will "color" all the sounds at any volume, unlike solid state. However, solid state in general have the quickest response time, perhaps even more so than modeling amps.

High-end solid state amplifiers are less common, since many professional guitarists tend to favor vacuum tubes. Some jazz guitarists, however, tend to favor the "colder" sound of solid-state amplifiers, preferring not to color the sound of their guitar with the tube distortion and compression so popular with rock, blues, and metal musicians.. Solid-state amplifiers vary in output power, functionality, size, price, and sound quality in a wide range, from practice amplifiers to professional models. Some purist or inexpensive amplifiers have only volume and tone controls.

Modeling amplifiers

Modeling amplifiers simulate the sound of well-known guitar amps, cabinets, and effects. This is usually achieved through digital processing, although there are analog modeling amps as well, such as the Tech 21 Sansamp (used by many known artists). Modeling technology offers several advantages over traditional amplification. A modeling amp typically is capable of a wide range of tones and effects, and offers cabinet simulation, so it can be recorded without a microphone. However, many guitarists consider modeling amps to be inferior to the amps they model, because the digital simulation fails to capture many nuances of a guitar amp's tone. This is partially due to the fact that speakers in Modeling amplifiers are often different to speakers in real amps and speakers are an important part of any amp's sound. Most modeling amps digitize the input signal and use a DSP, a dedicated microprocessor, to process the signal with digital computation.

Some modeling amps incorporate vacuum tubes, digital processing, and some form of power attenuation.

Amplifier configuration and set up

In the case of electric guitars, an amplifier stack consisting of a head atop one cabinet is commonly called a half stack, while a head atop two cabinets is referred to as a full stack. The cabinet which 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 8x12 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 4x12 cabinets, meaning four 12" speakers, to enable transporting the amp rig.

In heavy metal bands, the term "double stack" is sometimes used to refer to two stacks, with a second amplifier head serving as a slave to the first and four speaker cabinets in total. Another name for the "Head & Cab" that comes from the 1960s and 1970s is "Piggyback". Vox amp stacks could be put on a tiltable frame with casters. Fender heads could be attached to the cab and had "Tilt-Back" legs, like those used on larger Fender combo amps.

Typically, a guitar amp's preamplifier section provides sufficient gain so that an instrument can be connected directly to its input, and sufficient power to connect loudspeakers directly to its output, both without requiring extra amplification.

Another arrangement, often used for public address amplifier systems, is to provide two stages of amplification in separate units. First a preamplifier or mixer is used to boost the instrument output, normally to line level, and perhaps to mix signals from several instruments. The output from this preamplifier is then connected to the input of a power amplifier, which powers the loudspeakers.

Performing musicians that use the "two-stage" approach (as opposed to an amplifier with an integrated preamplifer and power amplifier) often want to custom-design a combination of equipment that best suits their musical or technical needs, and gives them more tonal and technical options. Some musicians require preamps that include specific features. Acoustic performers sometimes require preamps with "notch" filters (to prevent feedback), reverb, an XLR DI output, or parametric equalization. Hard rock, metal, or punk performers may desire a preamplifier with a range of distortion effects. As well, some musicians have specific power amplifier requirements, such as low-noise design, very high wattage, the inclusion of limiter features to prevent distortion and speaker damage, or biamp-capable operation.

With the "two-stage" approach, the preamplifier and power amplifier are often mounted together in a rack case. This case may be either free-standing or placed on top of a loudspeaker cabinet. If many rack-mounted effects are used, the rack may be a large unit on wheels. Some touring players need several racks of effects units to reproduce on stage the sounds they have produced in the studio.

On the other extreme, if a small rack case containing both preamp and power amp is placed on top of a guitar speaker cabinet, the distinction between a rack and a traditional amp head begins to blur. Another variation is to combine the power amplifier into the speaker cabinet, an arrangement called a powered speaker, and use these with a separate preamp, sometimes combined into an effects pedal board or floor preamp/processor.

Preamplifiers are also used to connect very low-output or high-impedance instruments to instrument amplifiers. When piezoelectric transducers are used on upright bass or other acoustic instruments, the signal coming directly from the transducer is often too weak and it does not have the correct impedance for direct connection to an instrument amplifier. Small, battery-powered preamps are often used with acoustic instruments to resolve these problems.

Distortion, power, and volume

Power output

For electric guitar amplifiers, there is often a distinction between "practice" or "recording studio" guitar amps, which tend to have output power ratings of 20 watts down to a small fraction of a watt, and "performance" amps, which are generally 50 watts or higher. Traditionally, these have been fixed-power amplifiers, with a few models having a half-power switch to slightly reduce the listening volume while preserving power-tube distortion.

The relationship between perceived volume and power output is not immediately obvious. A 5-watt amplifier is perceived to be 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. Doubling the power of an amplifier results in a "just noticeable" increase in volume, so a 100-watt amplifier is held to be 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 is being built into both low-power and high-power amplifiers, resulting in variable-power amplifiers. A high-power amplifier with flexible power attenuation built-in can produce power-tube distortion through the widest range of listening volumes. 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. Peak output of tube amplifiers is heard as being up to three times louder than similar rated solid state guitar amps. For example, a 30-watt tube amplifier can be perceived by the listener to be as loud as a 100-watt solid state amplifier, particularly when both are driven into maximum distortion.

Distortion and volume

Distortion is a distinguishing aspect of 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.

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, more power tube distortion is produced. This is slow-onset distortion. As such a tube (or valve) power amp rated at 50 watts produces 50 clean watts or, when pushed harder, twice as many distorted watts.

To make the amount of power-tube distortion independent from the volume heard at the guitar speaker, several approaches are used:

• The "power soak" approach places the attenuation between the power tubes and the guitar speaker. This pushes the power tubes to full power and then drains away the unwanted excess power by sending it to a mostly resistive dummy load and allowing only a portion through to the guitar speaker. Some power soak products include the Rockman Power Soak, the THD Hot Plate, the Weber MASS, and the Marshall Power Brake.
• The power-supply approach places the attenuation in the power supply and power amp; it runs the power tubes at a lower plate voltage and lets the full resulting power through to the guitar speaker. This prevents wearing-out the tubes or blowing the output transformer, and obviates purchasing and transporting a separate, bulky power attenuator. Some power supply attenuation circuits include London Power's Power Scaling, Maven Peal's Sag circuit, and Mojave's Power Dampening circuit.
• In the re-amped dummy load approach, the tube power amp drives a mostly resistive dummy load rather than a guitar speaker. The dummy load is typically the internal dummy load inside a power attenuator, with the load selector switched all the way to internal load rather than a mix of internal load and guitar speaker. A line-level signal is tapped from the dummy load, optional signal processing (such as equalization and reverb) is applied, and then the signal is sent through the final amplifier, typically solid-state run only in the linear region, which finally drives a guitar speaker. The guitar speaker contributes complex physical transducer dynamics at a quiet listening level controlled by the final amp, or even more complex dynamics if the speaker is pushed into distortion.
• 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. The guitar speaker and microphones are placed inside a soundproofed space separate from the control room or listening area, such as in a double-walled isolation room in a different floor of a house. A speaker cable runs from the tube guitar amplifier in the control room out to the miked guitar speaker cabinet out in the separate isolated area. The microphone cables then run back from the isolated area back into the control room. Professional recording studios often use this approach to obtain power-tube distortion independently of the listening volume.

Types of volume and gain controls

A variety of labels are used for level attenuation potentiometers in a guitar amplifier and other guitar equipment:

• Electric guitars and basses have a volume control to attenuate whichever pickup is selected. There may be two volume controls 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 distortion effect pedals have a volume and a distortion control. The volume control is a potentiometer at the output jack of the distortion pedal. The distortion control affects the driver amplitude prior to the clipping stages, so it affects the amount of clipping much more than it affects the output level. However, some distortion controls also change the amount of bass or treble. More complex distortion effect pedals may also add effects such as delay, additional harmonics, or octave generation.
• The simplest guitar amplifiers have only a volume control. Most have at least a gain control and a master volume control. The gain control 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 amplifier's tone controls typically include passive bass and treble controls. In some cases, a midrange control is provided. 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 attenuation is controlled by a knob on the tube power amp, variously labelled "Wattage", "Power", "Scale", "Power Scale", or "Power Dampening".

See also

• List of guitar amp brands
• Power attenuator (guitar)
• Instrument amplifier
• Bass instrument amplification
• Electronic amplifier
• Valve sound
• Guitar effects
• Guitar speaker cabinet
• Re-amp

External links

• Duncan's amp pages — All sorts of information, especially on tube guitar amplifiers.
• AX84 — Learn to build your own tube guitar amplifier with free schematics and plans.
• Choosing an Amp - What's a good guitar amp for a beginner?
• Amp Tube Charts and Transformer info - Amp tube charts and transformer chart for dating tube amps.
• Introduction to guitar amplifiers
• Introduction to bass guitar amplifiers
• Introduction to keyboard amplifiers
• List of books about guitar amplifiers and guitar amplifier tone
• Tons of Tones!! - Site for guitar amplifier modelling and patch libraries.
• SSGuitar.com - Information on building your own solid state amp/preamp/effects.
• Buyer of amps - Including a photo gallery of unusual and vintage amps.
• Tube Amps vs. Solid State Amps
• Guitar Amp modeling - Forum for guitar amp modeling software.