JTM45 British lead-style · 1962–1966 · 45 W

Marshall's first amplifier, and the first time one maker's circuit crossed to another: the JTM45 is a close copy of the tweed 5F6-A Bassman, rebuilt with British parts. Fender's low-gain 12AY7 input valve becomes an ECC83 (the European 12AX7), the 5881/6L6 output pair becomes a pair of KT66 beam tetrodes, the mains-supply voltages run a little higher, and solid-state diodes handle the bias supply. Everything downstream — the four inputs, the direct-coupled cathode follower feeding a treble-middle-bass tone stack, the long-tailed-pair phase inverter — is the Bassman, almost part for part. The higher-gain input valve and the KT66 bottles are what turned a clean bass amp into the seed of British rock tone. Produced from 1962; the drawing here is the mid-1960s revision shared across the tremolo combos and the 1987 head.

Circuit walkthrough (short form)

Bright + normal channels (1 MΩ leaks, 68 kΩ stoppers) → V1 ECC83 (100 kΩ plates, shared 820 Ω cathode with 250 µF bypass) → 0.02 µF couplers → 1 MΩ volume pots (100 pF bright cap) → 270 kΩ mixers → V3A ECC83 (100 kΩ plate, 820 Ω cathode) → V3B cathode follower, DC-coupled (100 kΩ cathode load) → TMB tone stack (56 kΩ slope; 270 pF, 0.02 µF and 0.01 µF caps; 250 kΩ / 1 MΩ / 25 kΩ pots) → 0.02 µF → long-tailed-pair PI: 82 kΩ and 100 kΩ plates, 470 Ω + 10 kΩ tail, both 1 MΩ grid leaks returned to the tail junction → 0.1 µF couplers → KT66 pair, fixed-biased through 220 kΩ leaks, with 33 kΩ grid stoppers and 1 kΩ · 2 W screen stoppers → output transformer, 27 kΩ negative feedback into the tail foot with the 5 kΩ presence control.

Power: 360-0-360 HT → GZ34 → standby → +450 V reservoir feeding the output plates (chart 430 V) → 20 H choke → +440 V screens → 8.2 kΩ → +380 V PI → 10 kΩ → +310 V preamp. Fixed bias: an HT-tap diode, 150 kΩ, and a 0.05 µF / 25 µF filter give the −50 V grid line.

The one part of the drawing not carried over here is the tremolo: on the factory sheet an extra ECC83 (its "V2") and a transistor form a bias-wobble oscillator that modulates the output valves. The plain head omits it, which is why the valve numbering skips from V1 to V3.

Lineage

The JTM45 is drawn from the 5F6-A, and the two schematics line up stage for stage. The differences are exactly the ones that give the JTM45 its voice: an ECC83 rather than a 12AY7 at the input (more front-end gain), KT66 output valves, 1 kΩ screen stoppers in place of Fender's 470 Ω, and a stiffer, higher HT rail.

Verification — against the printed factory chart

Simulation matches 11 of the chart's quantitative nodes — rails within 5.1 %, triode pins within 9.5 % worst case (the shared input cathode; the chart's own convention is ±20 %) — worst deviation 9.5 %. A twelfth value, the phase-inverter cathode (KPI), is excluded as disputed: the printed 40 V contradicts the chart's own 250 V plate figures (40 V across the 10 kΩ tail implies a current that would pull those plates far lower), and simulation — which reproduces the printed plates exactly — settles the cathode near 31 V, the same value the identical stage measures in the 5F6-A this circuit copies. The tail junction (JPI) carries no printed chart value at all — the drawing marks the PI grids only "+" — so it is reported informationally, not compared.