JMT45 Offset

The Marshall JTM45: A Sonic Evolution

The Marshall JTM45 was the pioneering guitar amplifier introduced by Marshall in 1963. It laid the groundwork for Marshall's iconic sound and became a cornerstone of rock music history.

Named after Jim and Terry Marshall, with "45" referring to its ambitious 45-watt RMS power rating.

Original JTM45 (1963)

Based on the Fender Bassman circuit

• 35-45 watts output power
• Initially used 5881 or 6L6 output tubes
• All-aluminium chassis
• 12AX7 preamp tube as the first in the chain
• Modified negative feedback circuit for unique harmonics
• Electronics initially placed at one end ("offset" design), later changed
• Initially built as a "head" with a separate 4×12" cabinet featuring Celestion speakers

Early Variants and Changes

• Switched to KT66 tubes in 1964, EL34 in 1966, and KT88 in 1967 for the 200-watt Major model
• Introduced "Coffin badge" models with red enamelled font on metal plates (rare, less than 100 made)
• All-black covering was introduced in late 1963

Production and Evolution

• Production of the original JTM45 Mk II model ceased in 1966, but it was reissued in 1989 with modifications.
• The JTM45/50 or JTM50 MK II (1965-1966) used KT66 output tubes and was rated at 45 watts.
• The EL34 variant, introduced in 1966, likely coincided with the transition to the "Plexi" era amplifiers.
• The KT88 variant, used in the 200W Major model, was also introduced in 1967.

Key Points:

The JTM45 underwent several changes during its early years, reflecting Marshall's adaptability to customer requests. The original JTM45 Offset was introduced in late 1962/early 1963.

Shortly after the amp's initial release, Marshall made some changes to the design, including wider input jack spacing and the elimination of a Bassman-style polarity switch on the front panel. 

With these changes, the amp was branded as the 'JTM 45 MK II'. While an exact date isn't provided, we can infer that the JTM45 Mk II was likely introduced sometime in 1963, very soon after the initial JTM45 release.

The transition to EL34 tubes marked a significant shift in Marshall's sound, contributing to the classic "British" tone associated with later Marshall amplifiers.

While the JTM45 designation may not have been explicitly used for all variants with EL34 and KT88 tubes, these amplifiers continued to evolve within the Marshall lineage.

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