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.
The tone network, as the drawing wires it
The Marshall drawing wires the stack exactly as the Fender 5F6-A sheet it
copies: the 270 pF treble capacitor and the 56 kΩ slope resistor both leave the
cathode-follower output; one 0.02 µF runs from the slope foot to the node
shared by the treble pot's lower lug and the bass pot; the bass pot is a
rheostat — the drawing loops its wiper to the foot lug — in series down to the
middle pot's top lug; the 0.01 µF mid capacitor feeds the middle pot's
wiper; and the stack's output is the treble pot's wiper alone, into the
0.02 µF phase-inverter coupler. The textbook redrawing of these parts joins the
treble and bass wipers at one output node and hangs the mid capacitor on a
rheostat-wired middle pot instead; the schematic, the board diagram and the
tone-stack lab all follow the drawing.
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.
