M1959 Super Lead 100‑style · 1965–1981 · 100 W

The Marshall model 1959 is the 100-watt Super Lead: the amplifier most people mean when they say "Plexi". It began as a special order — 100-watt heads built for players who wanted more than the 45-watt amplifier could give them on stage — and became Marshall's catalogue number 1959, produced from 1965 until 1981.

The signal path is the one the whole lead line shares — the 50-watt model 1987 carries the same one two years later. Four inputs feed two deliberately unmatched channels into one ECC83; a second ECC83 provides a gain stage and a direct-coupled cathode follower ahead of the treble-middle-bass tone stack; a third drives a long-tailed-pair phase inverter. What changes at 100 watts is behind the inverter: four EL34 pentodes in parallel push-pull, two per phase, over a solid-state bridge rectifier and a filter bank of paired 100 µF capacitors. There is no rectifier valve to sag, and there is a great deal of power supply behind the output valves, which is much of why the 1959 stays tight and loud where smaller amplifiers compress.

"Plexi" names the circuit and its voicing, not the panel: Marshall switched from the gold acrylic (Perspex) front panel to a brushed-metal panel around 1969, so the factory drawing cited here (Unicord, July 1970) is metal-panel era.

Circuit walkthrough (short form)

Two channels, each with high and low inputs (68 kΩ stoppers, 1 MΩ leaks) → V1 ECC83, one triode per channel (100 kΩ plates). The channels are voiced apart at the cathode: the high-treble channel runs 820 Ω fully bypassed by 250 µF, the normal channel a colder 2.7 kΩ with a 0.68 µF partial bypass, and their coupling caps differ too (0.022 µF and 0.0022 µF). Each plate feeds its own 1 MΩ volume with a bright cap (0.005 µF and 500 pF), and the two channels mix through 470 kΩ resistors into V2A (100 kΩ plate, 820 Ω / 0.68 µF cathode) → V2B cathode follower, DC-coupled (100 kΩ load) → tone stack (33 kΩ slope; 500 pF treble into 250 kΩ, 0.022 µF bass into 1 MΩ, 0.022 µF middle into 25 kΩ) → long-tailed-pair phase inverter (V3: 82 kΩ plate on the driven side and 100 kΩ on the other, 470 Ω over a 10 kΩ tail, both 1 MΩ grid leaks returned to the tail junction, 47 pF across the plates).

From there the two phases split: each 0.022 µF coupler feeds a pair of EL34s through 5.6 kΩ grid stoppers, with one 120 kΩ grid leak per pair carrying the adjustable negative bias. Every screen grid has its own 1 kΩ stopper. The output transformer offers 16, 8 and 4 Ω taps, and negative feedback returns from the 8 Ω tap through a 47 kΩ resistor to the cold end of the inverter's tail, where a 5 kΩ presence control shunts the top of the feedback band to ground.

Power: a universal-primary mains transformer (110/120/200/225/245 V taps), a silicon bridge and a reservoir of 100 µF capacitors in series pairs with 56 kΩ sharing resistors. That rail feeds the output valves' plates and screens directly; a 20 kΩ / 1 W dropper takes it down to the phase inverter and two further 10 kΩ / 1 W droppers step it down again for the second stage and the input valve, each node filtered by a pair of 50 µF capacitors. The negative grid bias comes from its own winding on the mains transformer, one end grounded: through 27 kΩ into its own diode, then 8 µF, 15 kΩ and a second 8 µF at the bias node, which a 47 kΩ and a 27 kΩ trimmer bleed to ground.

Lineage

The 1959 descends from the JTM45, and through it from the tweed 5F6-A Bassman the JTM45 copied. The path runs through the 100-watt heads Marshall built in 1965 for players who wanted more clean headroom than 45 watts gave: the output section was doubled, and the KT66 beam tetrodes gave way to EL34 pentodes as KT66 costs rose. The valve rectifier went with them. The front end barely moved — four inputs, two channels, a DC-coupled cathode follower and a long-tailed-pair inverter all carry straight across from the JTM45.

The 50-watt model 1987 is this amplifier's sibling rather than its parent: the same front end and the same voicing with half the output valves.

The tone network, as the drawing wires it

The Unicord drawing wires the stack identically to the model 1987's, wire for wire: 500 pF and the 33 kΩ slope off the cathode-follower output; 0.022 µF from the slope foot to the treble pot's lower lug and the bass pot; the bass pot a rheostat in series down the ladder; 0.022 µF into the middle pot's wiper; output from the treble pot's wiper alone. The schematic, the board diagram and the tone-stack lab all follow the drawing rather than the textbook redrawing that joins the two wipers.

A note on verification

Marshall drawings of this period print component values only, and the 1959 drawing carries no valve-voltage chart. A factory chart does exist for the 100-watt head, issued with the sheet of the same drawing number and date that covers the 6550-fitted Mark II — a closely related front end ahead of a different output quartet. Its ECC83 figures are used here for the preamp and phase inverter. The EL34 drawing places the 10 kΩ tail above a 5 kΩ presence track in parallel with the 47 kΩ feedback resistor, and a sharp (600 ppi) copy of the Mark II drawing returns its tail the same way: the 5 kΩ track's lower lug is grounded, with a 4.7 kΩ and a capacitor from the wiper to ground. Those drawings define the circuit; the chart cannot establish a different tail connection.

The inverter cathode simulates about 36.5 V against the Mark II chart's 18.5 V. The chart's own plate figures carry 1.77 mA, which gives exactly 18.5 V only through a 10 kΩ tail returned straight to ground — a connection neither drawing shows — so that figure is recorded as printed and marked disputed, and the circuit stays a draft. The EL34 output stage also has no published figures of its own: its plate and screen voltages here are derived from a rail taken as an assumption, and the idle current is a choice within the range these amplifiers are set to rather than a reading. Verified is earned against measurements, and the measurements that exist do not cover this output stage.