History · British-style

The Marshall lead-amp line

Marshall's first amplifier adapted a Fender tweed circuit for a British market wanting more volume, then diverged from it fast: within a few years the line had scaled from 45 to 100 watts, swapped its output valve and rectifier for louder, more available parts, and traded its acrylic ("plexi") front panel for metal. By the mid-1970s the founding hand-wired designs had been re-labelled and moved to circuit boards, closing out the era this family file documents.

1962–1981 5 models 2 documented 3 history-tier

Era timeline — each bar spans a model's production years; the chain from top to bottom is the line's succession.

Documented circuit — links to its page History-tier — placed & sourced, circuit not yet drawn Succession within the line

The models

Designation	Years	Watts	Output stage	Key change	Circuit
JTM45	1962–1966	45 W	push-pull 2xKT66	Marshall's first amplifier, built under Jim Marshall's direction and adapted from the circuit of a Fender tweed 5F6-A Bassman with higher-gain ECC83 preamp valves and added output filtering; a tube-rectified GZ34 supply. The corpus's own jtm45 entry documents the KT66-loaded variant of the model — other valve complements (5881, later 6L6GC) are reported for the same badge across its production run but are not the circuit modeled here.	JTM45 circuit ↗
JTM100	1965–1966	100 W	push-pull 4xKT66	Built at the request of The Who's Pete Townshend and John Entwistle, who wanted more clean headroom than the 45-watt head could give; Marshall doubled the output section to four KT66s. Schematic-archive era charts index this circuit separately from the later cataloged "1959" model number, though sources do not agree on whether the two names describe one continuous circuit or successive revisions — treated here as the immediate predecessor to the 1959-numbered Super Lead.	history entry
1959	1965–1981	100 W	push-pull 4xKT66, later 4xEL34	Marshall's catalog number for the 100-watt Super Lead head. Its power valves moved from KT66 to Mullard EL34 as KT66 costs rose, in the same window (mid-to-late 1960s) the corpus's own circuit entries bracket for the family: the jtm45 entry documents a GZ34 tube-rectified supply as late as 1966, while the m1987 entry documents a fully solid-state supply by 1970 — the exact year the 100-watt head's rectifier crossed over is not pinned down by the sources used here. The acrylic ("plexi") front panel was replaced by brushed metal in 1969, and the model picked up a "mkII" suffix and "JMP" branding after Marshall moved to printed-circuit construction in the mid-1970s.	history entry
JTM50	1966–1967	50 W	push-pull 2xKT66	A 50-watt scaled-down sibling of the 100-watt Super Lead, indexed under this name in schematic-archive era charts just ahead of the "1987" catalog number being applied to the same wattage class in 1967.	history entry
1987	1967–1981	50 W	push-pull 2xEL34	Marshall's 50-watt counterpart to the 1959 Super Lead, released in 1967 already on EL34 output valves. The corpus's own m1987 circuit entry documents a July-1970 factory drawing with a fully solid-state full-wave rectifier — no rectifier valve — continuing the same cost/reliability-driven shift away from tube rectification seen on the 100-watt head. Like its 100-watt sibling it went from acrylic to metal front panel in 1969 and picked up "mkII"/"JMP" branding once Marshall moved to printed-circuit construction in the mid-1970s.	M1987 circuit ↗

A circuit ↗ badge links a fully documented circuit — redrawn schematic, verified operating point, parts list. A history entry is placed and sourced here but its circuit isn't drawn yet; contributions are welcome.

Dating & notes

This line's dating is better documented than the tweed families, but two gaps remain honest gaps rather than resolved facts: (1) sources used here do not pin an exact year for the 100-watt head's tube-to-silicon rectifier crossover, only a bracket (GZ34 documented 1966, silicon documented 1970); (2) the relationship between the "JTM100"/"JTM50" schematic-archive names and the "1959"/"1987" catalog numbers — same circuits under two naming schemes, or successive revisions — is not settled by the sources consulted. Only jtm45 and m1987 have documented circuit entries in this corpus at present; the JTM100, 1959, and JTM50 rows above are history-tier only (circuit_ref: null) until a circuit entry is built for them.

Sources

Every model above carries its own citations in the corpus data file. Browse the full source list for this line: history/families/marshall-lead.yaml.

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