Gibson built amplifiers to sell guitars, and it built them its own way. The GA line ran from the late 1930s to the late 1960s and its house style has almost nothing in common with Fullerton's: pentode input valves rather than twin triodes, paraphase phase splitters long after the long-tailed pair had won, cathode bias with no bias supply anywhere in the chassis, one tone control rather than a tone stack, and tremolo that swings a pentode's screen instead of gating the signal path. The GA-40 Les Paul is the line's landmark — the amplifier Gibson named after its own endorsee, catalogued from 1952 until the Les Paul agreement ended in late 1962 — and it is really three different amplifiers wearing one model number, which is why the corpus files it by the circuit on the drawing rather than by the badge on the cabinet. Gibson also went its own way on bottles: where the rest of the industry standardised on 6V6 and 6L6, the last Les Paul amplifiers left the factory with 7591s and 6EU7s.
| Designation | Years | Watts | Output stage | Key change | Circuit |
|---|---|---|---|---|---|
| GA-40 | 1952–1954 | — | push-pull 2×6V6GT | The amplifier Gibson launched with the Les Paul name, and a different machine from the one the corpus documents. Its own tube-location strip letters three octal 6SJ7 pentodes — Mic Amp, Instrument Amp and Driver — a 6SN7 covering both the phase inverter and the tremolo, and then THREE 6V6GTs: two on the output and a third doing the tremolo's modulating, with a 5V4G rectifier. The house traits are already here: cathode bias on the output pair, a pentode at the input, and a tremolo that modulates rather than gates. Unlike its successor the sheet letters full reference designators (R1–R39, C1–C20) and prints a component legend and the transformer numbers, but it prints no voltages at all. The upper end of this bracket is the year of the latest drawing cited here that still shows this circuit, not a dated changeover — no source cited here dates the changeover itself, which is why a year sits unclaimed between this row and the next. | history entry |
| GA-40 | 1956–1962 | 15 W | push-pull 2×6V6GT | The octals go and the circuit the corpus documents arrives: a 5879 sharp-cutoff pentode at the front of each channel, a 6SQ7 running the tremolo, a 12AX7 paraphase phase inverter and two 6V6GTs on one shared 200 Ω cathode resistor behind a 5Y3GT. The paraphase is balanced by a 470 kΩ / 7.5 kΩ divider hanging off the first output grid, whose ratio of 1/63.7 is what a 12AX7 on a 220 kΩ load gives back. The tone circuit is a single capacitor and a single control on the inverter's own anode — the amplifier has no tone stack — and channel 2 gets its voicing instead from a five-section ladder of 0.005 µF capacitors and 1 MΩ shunts. The tremolo, credited to Seth Lover, modulates channel 2's screen and supply rather than gating the signal, which is why the factory chart prints +32 V on that screen against +95 V on channel 1's. The drawing carries no date and no revision box, and published accounts distinguish more than one 5879 version, so this bracket runs from the earliest year the sources cited here put the 5879 front end to the last year the model was catalogued — an outer bound over the 5879 revisions, not a production record. | GA40 circuit ↗ |
| GA-40T | 1962–1963 | — | push-pull 2×7591 | The end of the line, and the point where Gibson stops sounding like the 1950s. Every bottle changes: 6EU7s and a 12AU7 in front, a pair of 7591s on the output behind a GZ34, and — for the first time in this family — FIXED bias, with a −20 V supply and a gain-set control instead of one shared cathode resistor. The tremolo stops modulating a screen and becomes an optical circuit, a light-dependent resistor and bulb assembly in a can, switched by a foot pedal. The drawing's own legend names the parts by Gibson stock number down to the pots and the LDR. It is undated; the bracket here is the closing years of the Les Paul line rather than a dated changeover, and it is an outer bound on the sources cited below. | history entry |
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.
Every model above carries its own citations in the corpus data file. Browse the full source list for this line: history/families/gibson-ga.yaml.