Marshall JTM45 Schematics

The very first (prototype) Marshall amps were built in 1962 in Jims kitchen. In these early years the amps, their circuits and especially the cosmetics varied a lot.

Early 1965 saw the intro­duction of models 1961 and 1962. These were the very first Marshall combos.

JTM45, 45W head

This was the very first Marshall amp. It was built from 1962 up to 1966. It didn’t actually have a model name or number at first, but later it was called the JTM45.

The circuit was a 1959 Fender Tweed Bassman (circuit 5F6-A) clone. It is said that the first prototype and early production amps were fitted with two 5881 beam power tetrodes, three ECC83/12AX7 tubes and a GZ34 rectifier tube, but soon KT66 tubes were used instead of the 5881 tubes.

The Fender Bassman used a 12AY7 as the input tube but the JTM45 was fitted with a ECC83/12AX7 which has a higher gain. The mix resistors (which mix together both channels after the first triode sections) were originally 270kΩ on the early models. Sometime (late) in 1966 these mix resistors were raised in value to 470kΩ.

These amps had 2 channels with each a volume control and two inputs (making a total of four inputs). Next came the low, mid, high & presence controls. There was also a power switch and a standby switch with a power indicator.

The first JTM45 amps had no model number printed on them. Later (probably in 1963) aluminium panels were used having JTM 45 and MK II printed on them (implying the first serie of amps were MK I).

Schematic of the Bassman Tweed 5F6-A Amp (Fender).

JTM45 1961 Lead, 45W 4×10″ combo

This amp used the 1987 tremolo chassis in a 4×10″ combo cabinet.

Schematic with 4x ECC83, 2x KT66 & GZ34 (Marshall).

JTM45 1962 Bass & Lead, 45W 2×12″ combo (Blues Breaker)

This amp used the 1986 bass tremolo chassis in a 2×12″ combo cabinet. This was the amp Eric Clapton used on the legendary John Mayall’s Bluesbreakers “Beano” album early in 1966.

Note that the typical 100pF bright cap wasn’t fitted on the earliest models.

Schematic with 4x ECC83, 2x KT66 & GZ34 (Marshall).

JTM45 1963 Super PA, 45W head

This amp was in production from 1965 to 1966. It had 4 channels and 8 inputs. These PA amps had JTM 45 and MK III printed on the front panels.

Schematic with 4x ECC83 & 2x KT66 (Marshall).

JTM45 1985 PA, 45W head

This amp was in production from 1965 to 1966. It had 2 channels and 4 inputs. It was the PA version of the 1987.

JTM45 1986 Bass, 45W head

This amp was in production from 1965 to 1966. It had 2 channels and 4 inputs. It was the Bass version of the 1987.

JTM45 1987 Lead, 45W head

This amp was in production from 1965 to 1966. The original JTM45 was renamed to this model 1987. It had 2 channels and 4 inputs.

Photograph of a JTM45 1987 from 1966.

JTM45 1987T Tremolo Lead, 45W head

This amp was in production from 1965 to 1966. It had 2 channels and 4 inputs.

Schematic with 4x ECC83, 2x KT66 & GZ34 (Marshall).

JTM45 1989 Organ, 45W head

This amp was in production from 1965 to 1966. It had 2 channels and 4 inputs. It was the Organ version of the 1987.

1960, 4×12″ 60W cabinet

These first speaker cabinets didn’t have a model number at first. In 1964 these cabinets were given the model number 1960. They were angled front cabinets and were fitted with four 12″ Celestion G12-15 speakers, instead of the four 10″ Jensen speakers in an open cabinet like Fender did. These 12″ AlNiCo speakers were essentially the same speaker as the Celestion VOX Blue speakers. Late 1965 or early 1966 the ceramic speakers were replacing the AlNiCos.

1972, 2×12″ 50W extension cabinet

Very few of the 1972 2×12″ (horizontal) extension cabi­nets (for the JTM45 1962 amp) were built and these cabinets are very rare. The speakers were two 12″ Celestion G12-20 speakers. These G12-20 speakers were ‘M’ ceramic magnet speakers with an H1777 cone. These are not the small magnet G12-20s, but are actually 25W Greenbacks with a different cone, sounding different than a typical 25W Greenback from a 1960s 4×12″ cab.