AB763 (Twin Reverb-style) Blackface Twin Reverb‑style · 1963–1967 · 85 W

Fender's flagship of the blackface years and the reference for a loud clean amplifier: 85 watts from four 6L6GC output tubes into a pair of 12-inch speakers, with the same spring reverb and tube tremolo the smaller reverb amps carry. The circuit ran from 1963 to 1967. Two channels — a plain Normal and a Vibrato channel with the reverb and the tremolo — each get a full three-knob tone stack and a switched bright cap, and both feed one long-tailed pair driving the output quartet. There is no rectifier tube: the high tension is rectified by silicon, three series diodes to a leg.

The preamp bottles are marked 7025 on the drawing, the low-noise selected version of the 12AX7; the reverb driver and the phase inverter are 12AT7s, chosen for the current they can deliver, and the tremolo runs on its own 12AX7.

Signal path

Both channels, up to the volume control. Two inputs (68 kΩ stoppers into the grid, with the 1 MΩ leak at input 1's tip, jack side of its stopper) → a 12AX7 stage (100 kΩ plate load, 1.5 kΩ cathode with a 25 µF bypass) → a three-knob tone stack — 250 pF treble capacitor and 100 kΩ slope resistor both leaving the plate node, 0.1 µF to the node shared by the treble pot's lower lug and the bass pot, 0.047 µF down to the 10 kΩ middle control, and the treble wiper alone as the output — → a 1 MΩ volume with a 120 pF bright cap across it on a switch.

Normal channel. Volume → a second 12AX7 stage (100 kΩ plate) → a 0.047 µF coupler into a 220 kΩ mixing resistor. No reverb, no tremolo.

Vibrato channel. Volume → a second 12AX7 stage (100 kΩ plate) → a 0.02 µF coupler onto the dry node, which feeds both the reverb send and the mix driver.

Two cathodes, four triodes

The drawing marks nodes that appear in more than one place with a boxed letter, and two of those letters carry cathodes rather than supply rails. Each ties a pair of stages onto one 820 Ω resistor and one 25 µF bypass can:

• the Normal channel's second stage and the Vibrato channel's second stage;
• the reverb-recovery stage and the reverb/dry mix driver.

Both ends of each pair are printed at the same +2.0 V, which is what a shared node means. The consequence is worth stating plainly: the two stages on a shared resistor are not independent. Each one's current sets the other's bias, so the two are half as deeply biased as a private 820 Ω would make them, and anything that changes one — a swapped tube, a hotter section — moves the other with it.

Reverb and tremolo

Reverb. A 500 pF capacitor sends the vibrato channel's signal to a 12AT7 with both triodes in parallel (1 MΩ grid leak, 2.2 kΩ shared cathode) feeding the 125A20B transformer and the spring tank. What comes back is recovered by a 12AX7 stage across a 220 kΩ tank leak, coupled out by 0.003 µF, and blended with the dry signal by the 100 kΩ Reverb control through a 470 kΩ resistor onto the mix driver's grid.

Tremolo. The tremolo tube is a 12AX7 doing two jobs. One triode is a phase-shift oscillator (220 kΩ plate load, 2.7 kΩ cathode). Its ladder runs from the plate through 0.02 µF to the Speed node — a 3 MΩ Speed control and 100 kΩ to ground — then through 0.01 µF to a middle node and another 0.01 µF to the grid. A 1 MΩ from the middle node and another from the grid meet at the footswitch junction, which returns through 2.2 MΩ to the bias supply's filter node. The other triode takes its grid from that middle node and drives the neon lamp inside an optocoupler, sitting at the top of a 100 kΩ feed from the supply with a 10 MΩ bleeder across its plate. The lamp faces a photoresistor that shunts the wiper of the 50 kΩ Intensity control to ground. The control hangs from the mix driver's output, between its 0.1 µF coupler and its 220 kΩ mixing resistor, so the tremolo swings the level the vibrato channel hands to the phase inverter.

Mixing and inversion. The mix driver's grid is fed from the dry node through 3.3 MΩ with 10 pF across it, takes the reverb return through 470 kΩ, and is held at ground by a 220 kΩ leak. Its plate (100 kΩ load) is coupled out by 0.1 µF — the node the Intensity control hangs from — into the vibrato channel's own 220 kΩ mixing resistor. The two channels' mixing resistors meet, and a single 0.001 µF capacitor carries the sum to the phase inverter's grid.

Phase inverter and output. A 12AT7 long-tailed pair — 82 kΩ and 100 kΩ 5% plate loads, a 470 Ω cathode resistor down to a tail junction, 22 kΩ from that junction onward, and both 1 MΩ grid leaks returned to it — splits the signal for the four 6L6GC. The tail does not run straight to ground: it lands on the negative-feedback node, where an 820 Ω resistor comes back from the speaker and a 100 Ω leg goes to ground, so the feedback arrives in the tail. The output tubes are fixed-biased at −52 V through two 220 kΩ leaks — one per phase, each feeding two tubes — with a 1.5 kΩ stopper on every grid and a 470 Ω · 1 W resistor on every screen.

Power

340-0-340 V (power transformer 125P34A) → a silicon full-wave rectifier, three series diodes to a leg → the standby switch → a reservoir of two 70 µF · 350 V cans in series, balanced by a pair of 220 kΩ · 1 W resistors → +460 V at the output-transformer centre tap and the 6L6GC plates → the 125C1A choke → +458 V at the screens, the reverb-transformer feed and the tremolo → a 1 kΩ · 1 W dropper → +450 V at the phase-inverter plates → a 4.7 kΩ · 1 W dropper → +410 V feeding all six preamp plate loads, on 20 µF · 525 V cans. The grid line comes from the transformer's bias tap through a 470 Ω · 1 W resistor, a rectifier and a 25 µF · 50 V can, to a 10 kΩ balance control over a 27 kΩ leg whose wiper sets the −52 V.

The AB763 designation, twice

Fender put "AB763" on more than one drawing. The blackface Deluxe Reverb carries it too, and the two amps share the generation's recipe — fixed bias, a long-tailed-pair inverter, tube reverb and tremolo, the same tone-stack ladder — but they are not the same circuit and neither is descended from the other. The Deluxe Reverb is a cathode-cool 22-watt amp on two 6V6GTs, a GZ34 rectifier and two-knob tone stacks; this is an 85-watt amp on four 6L6GCs, silicon rectification, three-knob stacks, and a preamp with two extra gain stages to feed them. The archive keeps the bare ab763 for the Deluxe Reverb, which reached it first, and qualifies this one by its model.

The circuit this amp actually descends from is the blonde 6G8-A Twin — four bottles, fixed bias, solid-state rectification and a long-tailed pair were all in place before the blackface redesign added the reverb and the middle controls.

Reading against the printed chart

The drawing prints a full voltage chart, every value set at ±20 %, read to ground with an electronic voltmeter. Two nodes are held at their printed values in simulation — the +460 V reservoir and the +458 V node after the choke — and everything below them is solved through the drawing's own droppers, so the two lower rails are a test of the chart rather than an input to it. They pass: the phase-inverter supply lands at +446 V against a printed +450 V and the preamp supply at +408 V against +410 V, which is the arithmetic of six 100 kΩ plate loads and a phase inverter agreeing with the resistors Fender specified.

Every node on the chart falls inside the drawing's own ±20 % across the modelled stages: the reverb driver, the four preamp stages, the recovery and mix driver, the phase inverter's +100 V cathodes and +98 V tail junction, and the screens and grids of the output quartet. The widest gap is the vibrato channel's second stage, whose plate simulates about 15 % above its printed +240 V — and that is the shared-cathode node showing itself, because the chart prints +270 V at the other end of the same 820 Ω and two stages on one resistor cannot in fact sit 30 V apart.

This circuit is published as verified: every node the chart prints for a modelled stage is compared, none is set aside as disputed, and the worst gap is the one described just above.

The tremolo tube is left out of the DC solution, both halves. The oscillator is a running phase-shift circuit whose printed +2.0 V cathode is a grid-leak-detected average rather than a quiescent point, and the lamp driver's printed +380 V plate and +5.6 V cathode are what a meter reads while the neon lamp fires and extinguishes. Neither is a static operating point, so both are reported here rather than fitted to the chart. Unlike the Deluxe Reverb's tremolo, this one hangs off the +458 V node rather than the preamp rail, so leaving it out shifts nothing else on the chart.

The output quartet is simulated on a clean-room 6L6GC model fitted to RCA's own tabulated Class A1 characteristics for that tube. It idles at about 30 mA a bottle on the +460 V rail — a little under half the tube's 30 W plate rating, and well inside its 500 V ceiling.