Aracom Amplifiers Crank it up! History of Early Handwired 1962-1973 Marshall Amplifiers

HISTORY OF EARLY MARSHALL AMPLIFIERS

HISTORY OF EARLY MARSHALL AMPLIFIERS

THE 1962-1973 MARSHALL HANDWIRED AMPLIFIERS

This article focuses on the early generation Marshall amplifiers, which were offered during 1962 to 1973. For the purpose of this article, the 1962-1973 handwired amplifiers are categorized into three sections:

· 1962-1964 Metal Paneled (pre-plexi)

· 1965-1968 Plexiglas Paneled “Plexi

· 1969-1973 Metal Paneled

Marshall amplifiers from this period were known for their simplicity in design and awesome tone. Many of the pre-plexi and plexi Marshall amplifiers have become collector items and are quite valuable now.

Marshall amplifiers during this period had similar uncluttered control layouts. They were simple straight forward amplifiers. Features such as an effect loop or a master volume control was not offered. A few models would later be offered with Tremolo as an option. Reverb was even a rarer option. The early generation Marshall amplifiers consisted of a simple circuit design that allowed the amp to provide a raw natural tone. The smooth distortion, generated mostly from the power amp tube section, had plenty of overtones and harmonics. Many modern amplifiers generate a good portion of their distortion from multiple preamp gain stages, which can sound “buzzy” and reduces the “touch” response of the amplifier. The extra “bells and whistles” found on some amplifiers, requires extra circuitry and wiring (or additional circuit board trace lengths) that increases signal loss, which often has a negative effect on the overall tone.

The Marshall amplifiers referred to in this article are the original vintage handwired 1962-1973 Marshall amplifiers and not Marshall’s more recent reissue handwired products.

THE FIRST YEARS OF MARSHALL AMPLIFIERS 1962-1964 METAL PANELED (Pre-Plexi)

In 1962, Jim Marshall along with Ken Bran began their work on the first Marshall amplifier, which took place in Marshall’s shop in England. The prototype units became known as the MK I, with subsequent production models labeled as MK II along with the JTM 45 markings. The JTM acronym represents Jim & Terry Marshall. The JTM 45 amplifier circuitry was a close copy to the popular 1959 Fender Tweed Bassman 5F6A circuit, and today, both the JTM 45 and the 1959 Fender Bassman are legendary amplifiers, each providing their own unique tone. The JTM amplifier was Marshall’s response to the Fender Bassman. At that time, the Bassman was quite expensive in England, since the Bassman was imported from the US to the UK. Back in 1962, Jim Marshall wouldn’t have known the JTM would become an all time favorite blues and classic rock amplifier.

The 1962-64 Marshall amplifiers had aluminum metal front and back face panels. The panels were finished in natural aluminum or painted white. While these amplifiers were referred to as JTM, the prototypes and early product models did not have any model designations or markings on their front panels. Later production units would have the MKII and JTM 45 markings on the front aluminum panel. Initially, the amplifiers were sold only as head cabinets, with optional matching speaker cabinets sold separately. By late 1964, Marshall offered their first combo amplifiers.

As the first generation Marshall Amplifiers evolved, component and cosmetic changes often took place; therefore there are a few different variations of these early amplifiers. Additionally, the component and/or design changes did not necessarily take place at the end or beginning of the calendar year.

1962-1964 Marshall Amplifiers in original condition are considered highly prized collector items in today’s market.

THE PLEXI (PLEXIGLAS PANELED) ERA 1965-1968

Continuing with the JTM 45 circuit design, in 1965, Jim Marshall changed the front and back panels from metal to Plexiglas - hence the name "Plexi". For a short period between mid-1964 and 1965, the JTM was provided with cream colored front and back Plexiglas panels, followed by a gold plexiglas front panel that included a cream plexiglas rear panel. Marshall then settled on gold front and back plexiglas panels, as the majority of the JTM amplifiers were equipped in this fashion.

Marshall amplifiers during the Plexi Era:

· JTM 45 with either 5881, KT66 or 6L6 output tubes and a GZ34 rectifier tube;

· JTM 50 with EL34 output tubes; a few amplifiers were offered with the GZ34 rectifier tube and the majority equipped with a solid state rectifier;

· JMP 50 with EL34 output tubes and with a solid state rectifier.

· JTM 45-100, a 100 watt amp, was initially equipped with KT66 output tubes and later switched to EL34 output tubes and solid state rectifier.

· 18 Watt with EL84 output tubes and a EZ81 rectifier tube.

· 20 Watt with EL84 output tubes and solid state rectifier.

Most versions of these amplifiers were sold as head cabinet, with a smaller quantity sold configured as combos. The most famous combo was the JTM 45 Tremolo 2x12 played by Eric Clapton when he was with John Mayall's Blues Breaker group; hence, this amplifier is referred to as the "Blues Breaker". During this period, the JTM 45 and JTM 50 were offered with and without tremolo, in both head and combo versions. The 18w was available with Tremolo and as a combo version only.

Amplifier models offered during the 1965-1968 Plexi Era included the Lead (JTM 45 and 50), the Super Lead (100 watt), the Marshall Major (200 watts), a Bass Model and a PA Model. The electronic component difference between Lead, Bass and P.A. versions within each series was not major. Only a few components within the tone circuitry had different values. Additionally, each model designation was not limited to one specific use. For example, Bass and PA models were often also used by guitarists. These models can also be easily converted from one model type to another, simply by altering a few tone circuit components.

In 1965, Marshall introduced the 18 watt combo. The 18 watt was equipped with a pair of EL84 output tubes. While the 18 watt does not offer the same amount of clean headroom of larger wattage amplifiers, this particular amplifier was very popular for those wanting the "cranked up tone" at a lower overall volume level. The 20 watt head was introduced in 1967, but has proven to be less popular than the other Marshall amplifiers built during this era.

From 1962 until 1966 all of the Marshall chassis were built using aluminum. In 1967, Marshall began offering most models with a steel chassis.

The JTM45 amplifiers from 1962 up to1966 were equipped with a GZ34 rectifier tube. Tube rectified amplifiers will "sag" when fully cranked up. This was the result of the rectifier’s internal resistance and the tube "trying" to quickly provide enough current as required by the amplifier. Many guitarists like the "feel" and compression of tube rectified amplifiers, while others like the faster reacting solid state rectified tube amps.

1965-1968 Plexi Era – Year of Introduction:

1965 JTM 45

· Plexiglas paneled - 1962-1964 with metal panels.

· Rated between 30 - 45 watts.

· GZ34 tube rectifier, (2) 6L6 (KT66/5881) output tubes.

· Tube driven Tremolo effect optional. Offered in both Head and Combo cabinets.

1965 JTM 45-100

· 100 watt rating, solid state rectified, initially (4) KT66 Power tubes, then in 1966 changed to EL34s.

· The 100 watt was available in the new "Large" box head, in order to provide enough air circulation for the 4 power amplifier tubes.

18 Watt Series

· EZ81 tube rectified with (2) EL84 output tubes.

· Tube driven Tremolo standard with tube driven Spring Reverb optional.

· First Marshall amplifier to have the Reverb option, although it is rare to find one so equipped. Combo cabinets offered: 1x12, 2x12 and 2x10. Head cabinet was not available.

· The 18 Watt amplifier was available only for a short period between mid 1965 until 1967.

JTM 50

· Basically a JTM 45 equipped with EL34 output tubes.

· During this period, the GZ34 rectifier tube was being phased out in favor of the solid state rectifier that provided a different "feel" to the amplifier’s response.

· JTM 50 can be found with either tube or solid state rectifiers.

· Tube driven Tremolo effect optional. Offered in both Head and Combo cabinets.

1967 Marshall Major (200 watt rating)

· Solid state rectified with (4) KT88 output tubes.

1967 20 Watt Series

· Solid state rectifier with (2) EL84 output tubes.

· Offered as head cabinet only.

· Solid state driven Tremolo option; no Reverb option.

1968 JMP (50 and 100)

· Marshall changed the model designations from JTM to JMP.

· JMP 50 and JMP 100 were equipped with EL34 output tubes and a solid state rectifier, and were basically the same amplifiers that would become the JMP metal panel amp which was later released in 1969.

· The 1968 JMP amplifiers had plexiglas panels.

· Production of metal panel JMP amplifiers began mid-1969.*

JMP 50 amplifiers were offered in both Head and Combo cabinets; tube driven Tremolo effect was optional.

The JMP 100 was only offered in the head cabinet version.

* When Marshall introduced their new large box head in 1969 for the 100 watt amp, they discontinued the small box head design all together. Therefore in 1969, Marshall transitioned over to the large box head for both the JMP 50 and JMP 100 amplifiers.

Small box JMP 50 heads are rarer and typically more valuable than the large box versions.

During this period, Marshall also manufactured the JTM/JMP style amplifiers under the Park and Kitchen-Marshall brand names.

The 1965-1968 Plexi amplifiers are quite a collector’s item these days and continue to appreciate in value.

THE METAL PANEL, HANDWIRED ERA 1969-1973 (JMP Models)

During mid-year 1969, the gold Plexiglas front and back panels were dropped in favor of brushed gold aluminum panels; hence the term "Metal Panel" JMP. The electronic circuit design remained close to the original "Plexi" circuitry, and the amplifiers continued to be handwired through 1973.

Models offered included the Lead (50 watt), the Super Lead (100 watt), a Bass Model and PA Model. JMP 50 amplifiers were offered in both head and combo configurations, with and without Tremolo effect. The JMP 100 amplifiers were available only in the head configuration.

All of the handwired metal panel JMP 100 amplifiers and the majority of the JMP 50 amplifiers were of the "large" box variety. In general, the Marshall amplifiers became "brighter" sounding through the years. While the JMP 50 watt electronic circuitry was similar to the JTM 45, the tone was quite different due; to the tuning of the circuit "brightness", the use of EL34 power amplifier tubes, solid state rectifier instead of a tube rectifier, and different electrical specifications for the both power and output transformers. Another key difference was the JMP 50 and JMP 100 amplifiers had a split biased preamp section for each channel, whereas the JTM 45 and JTM 100 had a common biased preamp section. This resulted in the JMP amplifiers having slightly different gain in the first preamp stage for each channel, whereas the JTM amplifiers had the same gain in the first preamp stage for each channel. Both the JTM and JMP amplifiers had one “bright” and one “dark” channel for added bass response.

The metal paneled JMP amplifiers were equipped with EL34 power tubes that provided the famous cranked up "Marshall Crunch". EL34 tubes are known for their compressed, tight, and mid-focused distortion, in comparison to the JTM amplifiers that were equipped with 6L6 (5881/KT66) tubes that provided a more open and less heavy distortion. The JMP set the standards for hard rock tone.

In 1972 the Marshall Amplifiers that were destined to the US were equipped with the 6550 (KT88) power amplifier output tubes. The 6550 tubes offered their own unique tone which is often favored by metal style guitarists. However, most guitarists prefer the tone of the EL34 tubes over the 6550 tubes. Marshall amplifiers that were originally equipped with the 6550 tubes can be converted back for EL34 use.

Marshall handwired metal paneled amplifiers:

JMP (50)

· Rated at 50 watts, solid state rectified and equipped with a pair of EL34 or 6550 (KT88) output tubes;

· Tremolo effect optional;

· Head and combo cabinet options.

JMP (100)

· Rated at 100 watts, solid state rectified and equipped with quad EL34 or 6550 (KT88) output tubes;

· Head cabinet only.

Other handwired metal panel amplifiers include; the 20 Watt Series and the 200 Watt Series. Both of these amplifiers were only offered in the head cabinet configuration.

The production of Marshall’s early generation handwired amplifiers lasted from their conception in 1962 through most of 1973. Marshall then began phasing in printed circuit board (PCB) designs and phasing out their handwired amplifiers by late 1973. By 1974, most models were of the printed circuit board (PCB) design, ending the early handwired era. The PCB versions of the JMP amps were offered through 1981. The 1969-73 Marshall handwired JMP is an all time favorite hard rock amplifier.

Marshall Model Numbers (1962-1973 handwired era)

Model No. Product

1958 2x10 Combo: 18watt, 20 watt

1959 Super Lead, JTM 100, JMP 100

1959T Tremolo Super Lead: JTM 100, JMP 100

1968 Super P.A.: JTM 100, JMP 100

1974 1x12 or 2x12 Combo: 18watt, 20 watt

1985 P.A.: JTM 45, JMP 50

1986 Bass: JTM 45, JMP 50

1987 Lead: JTM 45, JMP 50

1987T Lead, Tremolo: JTM 45, JMP 50

1989 Organ: JTM 45, JMP 50

1961 4x10 Combo: JTM 45, JMP 50

1962 2x12 Combo: JTM 45, JMP 50 “Blues Breaker”

1992 Super Bass: JTM 100, JMP 100

1992T Tremolo Super Bass: JTM 100, JMP 100

1960 4x12 Cabinet

1972 2x12 Cabinet

1982 4x12 Cabinet

* For those new to the Marshall model numbers, the year of production can be easily mistaken for the model number, since the model numbers all begin with 19xx.

Cabinet and Speakers

The cabinets and speakers contributed as much to the "Marshall" tone as the amplifier itself. While the JTM's electronic circuitry was modeled after the Fender Bassman circuitry, the JTM produced its own unique tone. Differences in tone between the US made Bassman and the British made JTM can be attributed to the components available in the U.K. such as - brand and type of British & European tubes, British manufactured transformers, and the various electronic components sourced in England. The Fender Bassman was mostly composed of US manufactured parts, therefore these amps would naturally sound different. Also playing a significant part in overall tone was the use of the Marshall 4x12 enclosed cabinets. The large fully closed back cabinet, equipped with Celestion 12 inch speakers, provided a thunderous low end response. The 4x12 speaker enclosures were available as a straight front bottom cabinet and a angled front top cabinet. The two cabinets, along with the Marshall head, became known as the "Marshall Stack".

Marshall being a British company used Celestion speakers manufactured in England. Most famous were the Celestion 12" speakers that were used in the Marshall speaker cabinets. The vintage Marshall/Celestion speakers are known as Pre-Rola and Rola.

Pre-Rola speakers refers to the pre 1970 Celstion speakers labeled as:

CELESTION LTD, THAMES DITTON, SURREY

Compared to the post 1970 Celestion speakers were labeled as:

ROLA CELESTION LTD., IPSWICH, SUFFOLK

The difference being manufactured in either the Thames Ditton, Surrey, England factory and labeled as CELESTION LTD. (pre -Rola) prior to 1970 or the Ispwich, Suffolk, England factory labeled as ROLA CELESTION LTD. (Rola) from 1970 onwards. Celestion speakers manufactured from either factory during this period were all under the same company, Rola Celestion Limited.

Today, the Pre-Rolas are typically more expensive following “the older the better” motto.

Marshall initially used the 12 inch Celestion speakers equipped with lightweight ALNICO magnets. During the mid-60s, Marshall switched to the Celestion speakers equipped with ceramic magnets. These 12 inch ceramic speakers were available with medium and heavy magnets and with cone material designated as 75 Hertz (Hz) or 55Hz. The 75Hz designed more for lead guitar use and the 55Hz designed more for, but not limited to bass guitar applications. The ceramic speakers were available during the Celestion pre-Rola and Rola eras.

Popular Celestion speaker models offered in Marshall cabinets during the JTM/JMP era includes:

G12 (ALNICO)

G12M – a medium weight Ceramic magnet.

G12H – a heavier weight Ceramic magnet.

The Ceramic G12M and G12H speakers are referred to as Greenbacks, this is due to their plastic Green colored magnet covers. The G12M and G12H speakers were also available with plastic Black and Grey colored magnet covers and are known as Blackbacks and Greybacks.

The current line of Celestion reissue Greenback speakers, as well as the majority of Celestion’s guitar speakers, are now made in China. A few of Celestion’s high end speakers such as the G12 Blue (ALINICO), G12 Gold (ALNICO) and Heritage Line (Ceramic) are still manufactured in England (UK).

Vacuum Tubes (Valves) Used in the Handwired Marshall Amplifiers

Preamp

12AX7 (7025, ECC83) - The 12AX7 (US Designation) is a very popular 9 pin twin triode preamp tube. This tube was used nearly exclusively throughout the preamp circuit in the early Marshall amps. The 12AX7 was used in the early gain stage(s), tone control stage and in the phase inverter circuit. The 7025 is a high grade version of the 12AX7. The European Designation for the 12AX7 is ECC83.

Power Amp Tubes

6L6 (KT66/5881) - The 6L6, KT66 and 5881 are from the same family of tubes. The majority of the JTM amplifiers were supplied with power amp tubes of this group. The 6L6 (US Designation) is a very popular power amp tube and is also found in many of the larger Fender amps. As used in the JTM 45 applications, a pair of 6L6GC tubes can deliver around 45 watts or slightly more.

The 5881 (US Designation) is an upgraded (Military version) of the 6L6 tube.

The KT66 (European Designation) falls in the same family as the 6L6. The KT66 was manufactured by GEC of England; however these tubes have not been in production by the original manufacturer for more than a couple of decades. The KT66 is technically known as a Beam Tetrode Tube. Many guitarist and audiophiles consider the original GEC KT66 tubes as the "holy grail" of power amp tubes. Today NOS GEC KT66 tubes command very high prices. Marshall rated the JTM with a pair of KT66s at 30 watts, however a fully cranked Marshall with a pair of GEC KT66s can put out closer to 40 watts. There are several current tube manufactures offering their version of the KT66 tube.

While the 6L6, 5881 and KT66 tubes are from the same family, each type provides its own unique tone. These tubes as used in the JTM amplifiers are known for their warm blues tones and excellent classic rock overdriven tone.

EL34 (KT77) – The EL34 (European Designation) was used in the JTM 50 amplifiers and throughout the JMP amplifiers series. The EL34 has been the primary power amp tube from the late 60s through today for the Marshall amplifiers. One major difference in tone between the JTM and the JMP amplifiers is the use of the EL34 tubes. These tubes provide a very tight distortion and the famous Marshall "crunch". A pair of EL34 tubes is typically rated at 50 watts; however 60 watts or more can be achieved. The EL34 is a popular choice for hard rock and heavy metal applications.

6550 (KT88) - Earlier JMP amplifiers delivered to the US were equipped with EL34 tubes, however later versions to the US were provided with the 6550 (US Designation) tubes. The 6550 provides a loud clean tone and is also popular in Bass amplifiers. When cranked up, the 6550 provides a very tight crunch tone, which is popular with heavy metal players.

EL84 (6BQ5) – The EL84 (European Designation) tube was used in the Marshall 18 and Marshall 20 watt amplifiers as well as the VOX AC15 and AC30. These tubes are very popular today in mid sized amplifiers. The EL84 provides a warm blues break up and very nice overdriven tone, with plenty of harmonics.

Rectifier Tubes

5AR4 (GZ34) – The 5AR4 (US Designation) is a very widely used rectifier tube and is found in the Marshall JTM amps, large wattage Fender amps, VOX amps, as well as many other tube amplifiers.

EZ81 - The EZ81 (European Designation) was used in the Marshall 18 watt amps and can also be found in the VOX AC15 and other amplifiers in the 20 watt range.

Marshall Transformers

The output transformer contributes to the overall tone of the amplifier. The primary impedance of the output transformer is typically selected to be within "ball park" of the vacuum tube manufacturer's recommendation. The power amplifier tubes transfer power most efficiently to the speakers when the primary impedance of the transformer matches the combined output tube’s impedance. Like speakers, vacuum tubes also have reactive output impedances. The vacuum tube’s impedance will vary according to the operating point of the tube; therefore, there is no exact "correct" primary impedance value. The tube manufacture specifies their recommended impedance based on the typical operating range of the tube.

The primary impedance value, type of wire used, core material type, winding design and wiring technique affect the overall tone of the amplifier.

JTM 45 – The JTM 45 was equipped with either a Drake or RS Spares brand output transformer. The Drake (p/n 784-103) has a primary impedance of 8,000 ohms. RS Spares offered output transformers with primary impedances of 6,800, 8,000 and 9,000 ohms. The RS Spares output transformer with a primary impedance of 6,800 ohms was a widely used in the JTM 45 amplfiiers.

The primary impedances were selected based on the JTM being equipped with the 6L6/KT66/5881 group of tubes. With different primary impedances and the fact that they were manufactured by two different companies, the Drake and RS Spares have different tonal qualities.

JTM 50 and JMP 50 – the EL34 equipped JTM and JMP 50 watt Marshall amplifiers were fitted with the Drake (p/n 784-139) output transformers that had a primary impedance 3,400 ohms.

JTM 100 and JMP 100 – The earlier JTM 100 amplifiers were equipped with two JTM 45 output transformers. Marshall then switched to a single Drake (p/n 784-084) output transformer wound for a primary impedance of 4,000 ohms.

Marshall also used RS Spares and Drake power transformers.

The earlier Marshall amplifiers used the RS Spare transformers. In 1965, Marshall began migrating to the Drake transformers. During the 1966 model year and onwards, Marshall handwired amplifiers were mostly equipped Drake transformers.

SUMMARY

For the purpose of this article, the 1962-1973 handwired Marshall amplifiers were categorized into three eras. They are grouped according their face plate panel material.

· 1962-1964 Metal Paneled (pre-plexi)

· 1965-1968 Plexiglas Paneled “Plexi

· 1969-1973 Metal Paneled

However, some people refer to JTM strictly as the JTM 45 model and plexi covering the JTM 100, JTM 50 and JMP models.

The Marshall JTM 45 amplifier changed the tone of` Rock and Roll. While being based on the 1959 Fender Bassman circuit, the JTM's tone was distinctively different. 1962-64 pre-plexi Marshall amplifiers today, in original condition, are rare and valuable.

The Plexi “Plexiglas” paneled Marshall amplifiers were available from 1965-1968. During this period Marshall expanded their models, which included the JTM 45, 50,100, an 18 watt and a 20 watt. Tremolo versions of the JTM 45, JTM50 and 18 watt amps were also offered.

The JTM 45 is an all-time favorite vintage amplifier. The type of power amplifier tubes contributed a lot to the JTM 45's tone. The JTM45 amplifiers were equipped with the 6L6 family of tubes, which also included the 5881 and KT66 power tubes. These tubes provided a very nice warm clean or broken up blues tone, and a "classic rock" distorted tone when pushed full out. The JTM 45 is an all time favorite blues and rock amp.

Many guitarists who liked the JTM 45 tone but wanted more output power, turned to the JTM 100, that had a power output rating of 100 watts.

Today the metal paneled, hand wired Marshall amplifiers from 1969-1973 are the most affordable of the early hand wired Marshall amplifiers. The JMP 50 and 100 watt amplifiers are both identified with the "Marshall Crunch". The JMP amplifiers that are equipped with either the EL34 or 6550 tubes are associated with hard rock and heavy metal tones. The solid state rectifier provided the JMP with more punch and much less sag as compared to the tube rectified JTM amplifiers. The JMP set the standards for hard rock tone.

The JTM and JMP are all time favorite guitar amplifiers, each with its own unique tone. The 6L6/5881/KT66 equipped JTM is favored for its blues and classic rock tones. The EL34 equipped JMP amplifier is a favorite for those seeking the heavier "Marshall Crunch".

The Marshall 18 watt combo utilizing a pair of EL84 power amp tubes, is very popular with the guitarist who want an excellent blues or rock tone at lower volume levels.

Common to all of these Marshall handwired amplifiers was the minimal amount of preamp gain stages. Minimal preamp stages, allows the clarity of the guitar's signal to pass less distorted through the preamp circuitry; therefore the distortion is generated by overdriving the power amp section. Many high gain amplifiers rely on overdriving multiple preamp sections, creating a lot of their distortion prior to the power amp section. Thus, power amp distortion vs. pre amp distortion. The awesome overdriven tone from these Marshall amplifiers, was generated mostly in the power tube section, providing a warm overdriven tone as compared to the “buzzy” tone from overdriven pre-amp tubes.

Also contributing to the tone in a positive way, was the lack of effects, or minimal use of effects, such as only a Tremolo circuit. Most effect circuits will alter the original tone of the amplifier. For example, Effects Loops requires modification to the original circuit by adding extra components and wiring, directly in series or in parallel with preamp path and typically one or two extra preamp gain stages is also added. The additional circuitry creates signal loss and alters the amplifier's original tone. In the case of creating ultimate tone, less circuitry and “straight forward” design is often better, as these Marshall amplifiers have proven.

Contributing a lot to Marshall's signature tone was the use of British Celestion speakers and the 4 x 12 Marshall enclosed cabinets. Today some of the Pre-Rola and Rola speakers, as used in the Marshall speaker cabinets, are collector items. There are many versions of these speakers available and to understand the different versions can involve a lot of research.

Nearly all models of Marshall amplifiers from the handwired era have become popular among collectors and players. Each model providing a unique tone of its own. The value of these handwired Marshall basically follows the era they are from, the older being more valuable.

Who comes to mind as guitarist who favored the early Marshalls?

Jimi Hendrix, Eric Clapton, Duane Allman, Jeff Beck, Pete Townshend & John Entwistle , Ritchie Blackmore, Eddie Van Halen, Randy Rhoads, Slash, Angus Young, Peter Frampton, Eric Johnson, Yngwie Malmsteen, and Zakk Wylde, to name just a few. Hard to argue with the tone produced by these legendary guitarist!

References:

The History of Marshall – Michael Doyle

Considered one of the best books on Marshall Amplifiers.

The Fender Bassman 5F6-A – Richard Kuehnel

In depth and highly technical review of the 5F6-A circuit. Includes a technical section on differences between the JTM45 and 5F6-A circuitry.