Ampeg SVT

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The Ampeg SVT is a bass guitar amplifier designed by Bill Hughes and Roger Cox for Ampeg and introduced in 1969. The SVT is a stand-alone amplifier or "head" as opposed to a "combo" unit comprising amp and speaker(s) in one cabinet, and was capable of 300 watts output at a time when most amplifiers could not exceed 100 watts output, making the SVT an important amp for bands playing music festivals and other large venues. The SVT has been through many design changes over the years but is still in production today. While the SVT could be used with any 300 watt, 2- or 4-ohm cabinet combination, Ampeg recommended that it be used with a pair of sealed 8x10" speaker enclosures because one cabinet could not handle the power of the SVT. It wasn't until 1980 that the speakers in the enclosures were updated to a power handling rating of 350 watts, allowing a player to use an SVT head with only one cabinet. [1]

Bass guitar Electric bass instrument

The bass guitar is a plucked string instrument similar in appearance and construction to an electric guitar, except with a longer neck and scale length, and four to six strings or courses.

Instrument amplifier

An instrument amplifier is an electronic device that converts the often barely audible or purely electronic signal of a musical instrument into a larger electronic signal to feed to a loudspeaker. An instrument amplifier is used with musical instruments such as an electric guitar, an electric bass, electric organ, synthesizers and drum machine to convert the signal from the pickup or other sound source into an electronic signal that has enough power, due to being routed through a power amplifier, capable of driving one or more loudspeaker that can be heard by the performers and audience.

Ampeg company

Ampeg is a manufacturer best known for its bass amplifiers. Originally established in 1946 in Linden, New Jersey by Everitt Hull and Stanley Michaels as "Michaels-Hull Electronic Labs," today Ampeg is part of the Yamaha Guitar Group. Although the company specializes in the production of bass amplifiers, it has previously manufactured guitar amplifiers, pickups and several instruments including, double basses, bass guitars, and electric guitars.


SVT originally stood for Super Vacuum Tube, but Ampeg has since revised the meaning of the acronym to Super Valve Technology , [2] with the word "valve" referring to the vacuum tubes (called "valves" in Britain and some other regions) used in the amp.

Vacuum tube Device that controls electric current between electrodes in an evacuated container

In electronics, a vacuum tube, an electron tube, or valve or, colloquially, a tube, is a device that controls electric current flow in a high vacuum between electrodes to which an electric potential difference has been applied.


Following Unimusic's acquisition of Ampeg in 1967, the new company management was actively pursuing the rock market, opening offices in Chicago, Nashville, and Hollywood, and developing products designed to address the needs of rock musicians. When The Rolling Stones began rehearsing for their 1969 U.S. Tour in Hollywood, a power conversion failure blew up all of their UK Fender amplifiers. Their road manager, Ian Stewart contacted Rich Mandella at the Ampeg office in Hollywood, and Rich arranged for the band to use five prototype high-output amplifier heads of a new model being developed by Bill Hughes and Roger Cox. These new amps employed a 14-tube design to generate 300 watts of power in an era when most tube amps generated less than 100. The Rolling Stones took these protoype Ampeg amps on tour along with Rich Mandella, playing all guitars and basses through them for the entire tour. After the tour, Ampeg put the SVT into production, introducing it at the NAMM Show in 1969. [3] [4] [5] [6] [7] [8]

The Rolling Stones English rock band

The Rolling Stones are an English rock band formed in London in 1962. The first stable line-up consisted of bandleader Brian Jones, Mick Jagger, Keith Richards, Bill Wyman (bass), Charlie Watts (drums), and Ian Stewart (piano). Stewart was removed from the official line-up in 1963 but continued to work with the band as a contracted musician until his death in 1985. The band's primary songwriters, Jagger and Richards, assumed leadership after Andrew Loog Oldham became the group's manager. Jones left the band less than a month before his death in 1969, having already been replaced by Mick Taylor, who remained until 1974. After Taylor left the band, Ronnie Wood took his place in 1975 and continues on guitar in tandem with Richards. Since Wyman's departure in 1993, Darryl Jones has served as touring bassist. The Stones have not had an official keyboardist since 1963, but have employed several musicians in that role, including Jack Nitzsche (1965–1971), Nicky Hopkins (1967–1982), Billy Preston (1971–1981), Ian McLagan (1978–1981), and Chuck Leavell (1982–present).

Ian Stewart (musician) British musician and tour manager

Ian Andrew Robert Stewart was a Scottish keyboardist and co-founder of the Rolling Stones. He was removed from the line-up in May 1963 at the request of manager Andrew Loog Oldham who felt he did not fit the band's image. He remained as road manager and pianist and was posthumously inducted into the Rock and Roll Hall of Fame along with the rest of the band in 1989.

The NAMM Show is an annual event in the US that its organizers describe as "the world’s largest trade-only event for the music products, pro audio and event tech industry".

Ampeg SVT Classic.jpg
Ampeg SVT Classic
(black face & grill)


There are three types of original SVT amps. The first are the "blue line" SVTs, so named after the blue screen printing that surrounds the tone controls. Early 1969-70 "blue lines" used 6146B beam power vacuum tubes in the output stage, which proved unstable and was switched to the more robust, reliable and commonly-used 6550 tube around mid-1970. [9]

Beam tetrode type of tetrode vacuum tube

A beam tetrode, sometimes called a "beam power tube", is a type of tetrode vacuum tube with auxiliary beam-focusing plates designed to augment power-handling capability and help reduce unwanted emission effects. These tubes are usually used for power amplification, especially at audio-frequency.

The second version of a vintage SVT is what is called the "black line" SVT, earning its name from the black (rather than blue) faceplate screen printing. Like the later-revision "blue lines" models, the "black line" SVTs utilize 6550 power vacuum tubes instead of 6146Bs. Later 1970s models have the same features as the "black line" SVTs, except the lines around the tone controls have rounded corners and curve into the tone controls. Additionally, these models included 3-prong power cables, and did not include a polarity switch.

Ampeg SVT VR "Vintage Reissue"   (blue line) AMPEG SVT VR (2).JPG
Ampeg SVT VR "Vintage Reissue"   (blue line)

In the early 1980s, Ampeg was bought by Music Technologies, Inc. (MTI), which contracted to have SVTs manufactured in Japan. While MTI-era SVTs are mostly identical to the previous versions, they did have differences. Cosemetically, MTI SVTs have black faceplates with white lettering, black grill cloth, "elephant hide" or rougher textured tolex, and rack case-style spring-loaded handles, updated from the previous (and painful) rubber-covered metal strap handles. These SVTs also include a back panel selector toggle for 2 or 4 ohm speaker impedance loads and a longer and thicker gauge 3-prong power cable. Additionally, some components, such as the transformers, on MTI-era SVTs are of Japanese origin as opposed to the original SVT transformers made by Chicago-based ETC..

In 1986, St. Louis Music acquired the rights to the Ampeg name and took possession of all remaining MTI inventory, which contained enough original components to build 500 amps. These 1987 Limited Edition SVTs were built in the U.S. by SLM's own Skunk Works crew, and each included an engraved panel indicating the unit's number within the production of 500 total units. [10] In 1990, Ampeg introduced the SVT-II and SVT-II Pro, and in 1994, introduced the SVT-CL (Classic).

St. Louis Music (SLM) is a manufacturer and distributor of musical instruments, accessories, and equipment. SLM distributes products from over 260 music products industry brands, and is the corporate owner of Austin Guitars, Knilling String Instruments, Hamilton Stands, SIGMA Guitars USA, Dixon Drums, and Zonda Instruments & Reeds. Additionally, SLM is the exclusive worldwide distributor and producer of Alvarez and Alvarez-Yairi guitars.

In 2005, LOUD Technologies (now LOUD Audio, LLC) acquired St. Louis Music, including Ampeg. Under LOUD's management, production of Ampeg and versions of SVTs and cabinets was moved to Asia. In 2010, Ampeg introduced the Heritage Series line, manufactured in LOUD Technologies' facility in Woodinville, Washington, including the Heritage SVT-CL head and SVT-810E and SVT-410HLF cabinets. The updated head featured JJ-branded preamp and driver tubes and "Winged C" 6550 power amp tubes, all tested and matched by Ruby Tubes in California, along with a thicker 1.6mm two-layer printed circuit board with through-hole plating and increased copper weight. [11]

In May, 2018, Yamaha Guitar Group acquired Ampeg from LOUD Audio. [12] Ampeg continues to manufacture and sell Heritage Series and SVT Pro Series models of SVT.

Tubes and modifications

The SVT amps with 6146B tubes tend to put out a bit more power as well as have a more pronounced grind in low mids, as opposed to the more round, deeper bass sound provided by SVTs with 6550A tubes. This sound characteristic is mostly due to 6146 being much lower in transconductance and less sensitive to drive signal, requiring a higher level from the Pre-amp, which can create more "harmonic growl."

The issues with the earliest version of the SVT were in the early design of the driver circuit, not the 6146B tube which is a stable and reliable tube. The earliest SVT driver circuit's design would on occasion result in blown 6146B tubes. Because the front end 12BH7 voltage amplifier is fed from the 430 V node, during loud transients and overloads this will produce an AC signal that far exceeds the 12BH7 follower that has 220 V on the plates. So when the follower grid is driven well over its own plate voltage, it saturates on the positive half of the signal and thus takes over the BIAS voltage, forcing it very positive. The time for this voltage to come back to normal is based on the time constant of the 150 kΩ mixer resistors and the coupling cap - but by this time it is too late, the bias is pushed too far into the positive and the current gets pushed through the 6146, resulting in blown tubes.

Some users have an electronics technician re-wire the 12BH7 feed the same as on the later 6550 heads. By adding a 1K and filter cap feeding from the 220 V screen supply to the front voltage amp of the 12BH7, a 6146B tube runs reliably with far fewer issues. However, conversion of 6146B to 6550 tubes also has a dramatic impact on the output power. The amp will produce roughly 225 watts, due to the screen voltage being too low. As such, the power transformer of a 6146B SVT will be about 220 V DC on the screen supply at idle instead of the typical 350 V idle screen voltage normally seen on 6550 amps. One solution is wire the screens in a voltage double arrangement, which will end up at roughly 400 V screen voltage at idle. This will make for a very powerful 400 W SVT. Conversion from a 6550 tube to 6146B tube is a bit trickier, as the 6146B will not tolerate anything over 250 V on the screens or else it will arc over. Some amp technicians prefer to disassemble to PT and tap the windings from the side of the bobbin to create a lower voltage taps. Other methods are to use voltage regulation. [13]

The SVT's pre-amp is notorious for ground loop hum. Re-wiring and separating the audio ground shield from the power return ground lead in the MOLEX connector is one solution. The pre-amp can sound quicker if the circuit is rebuilt with Hi-End Audio Grade coupling caps. Diodes were later used to by-pass the 22ohm screen resistors. In the event of a tube short failure or simply a transient overload condition the diode will conduct once the current in the 22ohm screen resistor reaches 30mA and beyond, preventing further burning of the PCBA. The diode will clamp the current in the 22ohm screen resistor to 30mA, so preferably the plate resistor will blow, since the plate resistor is acting as a "fuse".

Plate resistors should be kept off the circuit board by approx 1/2" min to prevent PCBA burning. The diode is taking on the current surge to protect the 22 Ω resistor, however the peak over current in the diode can only sustain for a short duration. The 6550A version usually idles with roughly 700 V DC on the plate and 350 V DC on the Screens (though during full clean sine wave 300 W power output, operating voltages will dip to roughly 650 V DC on the Plate and 325 V on the Screens). The output transformer plate load is 1.6 K at 4 Ω tap and 1.75 K at the 2 Ω tap.

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Audio power is the electrical power transferred from an audio amplifier to a loudspeaker, measured in watts. The electrical power delivered to the loudspeaker, together with its efficiency, determines the sound power generated.

Valve amplifier type of electronic amplifier

A valve amplifier or tube amplifier is a type of electronic amplifier that uses vacuum tubes to increase the amplitude or power of a signal. Low to medium power valve amplifiers for frequencies below the microwaves were largely replaced by solid state amplifiers during the 1960s and 1970s. Valve amplifiers are used for applications such as guitar amplifiers, satellite transponders such as DirecTV and GPS, audiophile stereo amplifiers, military applications and very high power radio and UHF television transmitters.

Damping factor

In an audio system, the damping factor gives the ratio of the rated impedance of the loudspeaker to the source impedance. Only the resistive part of the loudspeaker impedance is used. The amplifier output impedance is also assumed to be totally resistive. The source impedance includes the connecting cable impedance. The load impedance and the source impedance are shown in the diagram.

12AX7 A family of high-gain double triodes

12AX7 is a vacuum tube that is a miniature dual triode - 6AV6 with high voltage gain. It was developed around 1946 by RCA engineers in Camden, New Jersey, under developmental number A-4522. It was released for public sale under the 12AX7 identifier on September 15, 1947. The 12AX7 was originally intended as replacement for the 6SL7 family of dual-triode amplifier tubes for audio applications. It is popular with tube amplifier enthusiasts, and its ongoing use in such equipment makes it one of the few small-signal vacuum tubes in continuous production since it was introduced.

All American Five

The term All American Five is a colloquial name for mass-produced, superheterodyne radio receivers that used five vacuum tubes in their design. These radio sets were designed to receive amplitude modulation (AM) broadcasts in the medium wave band, and were manufactured in the United States from the mid-1930s until the early 1960s. By eliminating a power transformer, cost of the units was kept low; the same principle was later applied to television receivers. Variations in the design for lower cost, shortwave bands, better performance or special power supplies existed, although many sets used an identical set of vacuum tubes.

Mesa Boogie company

Mesa/Boogie is an American company in Petaluma, California, that manufactures amplifiers for guitars and basses. It has been in operation since 1969.

Headphone amplifier Audio amplifier used with headphones

A headphone amplifier is a low-powered audio amplifier designed particularly to drive headphones worn on or in the ears, instead of loudspeakers in speaker enclosures. Most commonly, headphone amplifiers are found embedded in electronic devices that have a headphone jack, such as integrated amplifiers, portable music players, and televisions. However, standalone units are used, especially in audiophile markets and in professional audio applications, such as music studios. Headphone amplifiers are available in consumer-grade models used by hi-fi enthusiasts and audiophiles and professional audio models, which are used in recording studios.

EL34 Audio freqency power pentode

The EL34 is a thermionic valve or vacuum tube of the power pentode type. It has an international octal base and is found mainly in the final output stages of audio amplification circuits and was designed to be suitable as a series regulator by virtue of its high permissible voltage between heater and cathode and other parameters. The American RETMA tube designation number for this tube is 6CA7. The USSR analog was 6P27S.

Single-ended triode A single-ended tube audio amplifier employing a triode in the output stage

A single-ended triode (SET) is a vacuum tube electronic amplifier that uses a single triode to produce an output, in contrast to a push-pull amplifier which uses a pair of devices with antiphase inputs to generate an output with the wanted signals added and the distortion components subtracted. Single-ended amplifiers normally operate in Class A; push-pull amplifiers can also operate in Classes AB or B without excessive net distortion, due to cancellation.

Bass amplifier

A bass amplifier or "bass amp" is a musical instrument electronic device that uses electrical power to make lower-pitched instruments such as the bass guitar or double bass loud enough to be heard by the performers and audience. Bass amps typically consist of a preamplifier, tone controls, a power amplifier and one or more loudspeakers ("drivers") in a cabinet.

Distortion (music) form of audio signal processing giving "fuzzy" sound

Distortion and overdrive are forms of audio signal processing used to alter the sound of amplified electric musical instruments, usually by increasing their gain, producing a "fuzzy", "growling", or "gritty" tone. Distortion is most commonly used with the electric guitar, but may also be used with other electric instruments such as bass guitar, electric piano, and Hammond organ. Guitarists playing electric blues originally obtained an overdriven sound by turning up their vacuum tube-powered guitar amplifiers to high volumes, which caused the signal to distort. While overdriven tube amps are still used to obtain overdrive in the 2010s, especially in genres like blues and rockabilly, a number of other ways to produce distortion have been developed since the 1960s, such as distortion effect pedals. The growling tone of distorted electric guitar is a key part of many genres, including blues and many rock music genres, notably hard rock, punk rock, hardcore punk, acid rock, and heavy metal music.

The KT88 is a beam tetrode/kinkless tetrode vacuum tube for audio amplification.

Epiphone Valve Junior

The Epiphone Valve Junior is a small 5 watt class A electric guitar amplifier.

Fender Champ

The Fender Champ was a guitar amplifier made by Fender. It was introduced in 1948 and discontinued in 1982. An updated version was introduced in 2006 as part of the "Vintage Modified" line.

Technical specifications and detailed information on the valve audio amplifier, including its development history.


Tung-Sol was an American manufacturer of electronics, mainly lamps and vacuum tubes.

Circlotron Symmetrical bridged topology of an output-transformerless (OTL) valve amplifier

Circlotron valve amplifier is a type of power amplifier utilizing symmetrical cathode-coupled bridge layout of the output stage. Original circlotrons of 1950s used output transformers to couple relatively high output impedance of vacuum tubes to low-impedance loudspeakers. Circlotron architecture, easily scalable, was eventually adapted to operate without output transformers, and present-day commercially produced circlotron models are of output transformerless (OTL) type.

Plate detector (radio)

In electronics, a plate detector is a vacuum tube circuit in which an amplifying tube having a control grid is operated in a non-linear region of its grid voltage versus plate current transfer characteristic near plate current cutoff in order to demodulate an amplitude modulated carrier signal. This differs from the grid leak detector, which utilizes non-linearity of the grid voltage versus grid current characteristic for demodulation. It also differs from the diode detector, which is a two terminal device.

Tube sound

Tube sound is the characteristic sound associated with a vacuum tube amplifier, a vacuum tube-based audio amplifier. At first, the concept of tube sound did not exist, because practically all electronic amplification of audio signals was done with vacuum tubes and other comparable methods were not known or used. After introduction of solid state amplifiers, tube sound appeared as the logical complement of transistor sound, which had some negative connotations due to crossover distortion in early transistor amplifiers. The audible significance of tube amplification on audio signals is a subject of continuing debate among audio enthusiasts.


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