AB763 Blackface Deluxe Reverb‑style · 1964–1967 · 22 W

The circuit most players picture when they think "Fender clean": a 22-watt, 6V6 combo with tube reverb and a genuine tremolo. The blackface Deluxe Reverb arrives in 1963 on the AA763 drawing; the AB763 revision below is the one that carried it from 1964 to 1967 and the one the model is remembered by. Two channels share the output stage — a plain Normal channel and a Vibrato channel carrying the reverb and the tremolo — feeding a fixed-biased 6V6GT pair through a 12AT7 phase inverter, rectified by a GZ34. The preamp bottles are marked 7025 on the drawing, the low-noise selected version of the 12AX7; the reverb driver and phase inverter are 12AT7s, chosen for the current they can deliver.

Signal path

Normal channel. Two inputs (68 kΩ stoppers into the grid, with the 1 MΩ leak at input 1's tip, jack side of its stopper) → first 12AX7 stage (100 kΩ plate load, 1.5 kΩ cathode with a 25 µF bypass) → a treble-bass tone stack (100 kΩ slope, 250 kΩ treble and bass, 6.8 kΩ bleed, and 250 pF · 0.1 µF · 0.047 µF caps) and a 1 MΩ volume → a second 12AX7 stage (100 kΩ plate load) → a 0.047 µF coupler and a 220 kΩ mixing resistor into the phase inverter. No reverb, no tremolo.

Vibrato channel. Input stage as above → the identical tone stack, part for part (with a 47 pF bright cap across the volume pot, top lug to wiper) and a 1 MΩ volume → a second 12AX7 stage (100 kΩ plate, 820 Ω cathode) → the reverb and tremolo section.

One cathode for two stages. The two channels' second stages share a single cathode network. The drawing marks both cathodes with a boxed A and draws the 820 Ω resistor and its 25 µF bypass once, at the Vibrato stage; the Normal stage has no cathode resistor of its own, and the factory layout runs its cathode pin to the Vibrato bottle's before the one lead goes on to the 820 Ω. That resistor carries both triodes' current, and the +1.3 V the chart prints at each end of the box is a single point.

The tone stacks, as the sheet wires them

Both channels draw the same two-knob ladder the blackface AA964 prints — not the textbook redrawing of these parts — and the schematic here follows the sheet (re-read at lug level 2026-08-03):

• The 250 pF treble capacitor and the 100 kΩ slope resistor both leave the plate node; the three capacitors do all the DC blocking.
• The 0.1 µF runs from the slope foot to the node shared by the treble pot's lower lug and the bass pot — the treble pot's cold end sits on the far side of that capacitor, not on the slope foot.
• The bass pot is a rheostat above the fixed leg, and the 0.047 µF runs from the slope foot to the bass pot's foot, where the 6.8 kΩ bleed takes the network to ground.
• The stack's output is the treble pot's wiper alone, into the volume control. The tone-stack lab plots the normal channel with this wiring.

Reverb. The dry vibrato signal drives a 12AT7 with both triodes in parallel (2.2 kΩ shared cathode) into the 125A20B transformer and the spring tank. The returned signal comes back through a 12AX7 recovery stage (100 kΩ plate, 820 Ω cathode) and meets the dry signal at the mix driver's grid: the reverb through 470 kΩ from the 100 kΩ Reverb control's wiper, the dry signal through 3.3 MΩ with 10 pF across it, and a 220 kΩ to ground holding the grid. The mix driver has no cathode resistor of its own either: it shares the recovery stage's 820 Ω and 25 µF, a connection the drawing marks with a boxed E.

Tremolo. One half of a 12AX7 is a phase-shift oscillator: its plate feeds a 0.02 µF / 0.01 µF / 0.01 µF ladder back to its own grid, and the 3 MΩ Speed control, in series with 100 kΩ, tunes the ladder's middle to ground. The other half, coupled directly to the ladder, drives a neon lamp in its plate circuit. The lamp faces a photoresistor inside an optocoupler; the photoresistor hangs from the Intensity control's wiper, and Intensity's other end takes the vibrato channel's output just after the mix driver, so each flash shunts that signal toward ground and the volume swings. The vibrato footswitch grounds the oscillator's feed and stops it.

Phase inverter and output. The two channels meet at one node, each through its own 220 kΩ mixing resistor — the Vibrato side after the mix driver's 0.1 µF coupler — and a 0.001 µF capacitor carries that node into a 12AT7 long-tailed pair (82 kΩ and 100 kΩ 5% plate loads, a 470 Ω cathode resistor to a tail junction, a 22 kΩ tail from that junction to the feedback node, both 1 MΩ grid leaks returned to the junction) splits the signal for the 6V6GT pair. The output tubes are fixed-biased at −35 V through 220 kΩ leaks, with 1.5 kΩ grid stoppers mounted on the sockets and 470 Ω · 1 W screen resistors, and an 820 Ω negative-feedback resistor returns from the speaker to the foot of the inverter's tail, where a 47 Ω goes to ground: the tail current and the feedback share that node.

Power

330-0-330 V (power transformer 125P33A) → GZ34 → +415 V at the 6V6 plates (the output transformer centre tap sits at +420 V) → filter choke → +415 V screens → a 10 kΩ dropper → +325 V at the phase-inverter plates → a second 10 kΩ dropper → the preamp rail every 100 kΩ-loaded 12AX7 stage hangs from — both channels' input and second stages, the reverb recovery and the mix driver — which the chart gives no voltage of its own. A separate negative supply — a 470 Ω · 1 W feed, a rectifier, one 25 µF · 50 V can and a 10 kΩ divider — provides the −35 V fixed bias.

Bias and lineage

Where the tweed Deluxe cathode-biased its 6V6s, the blackface Deluxe Reverb runs a proper negative-bias supply — the recipe it inherited from the brownface Deluxe (the 6G3) rather than from any tweed, along with that amp's long-tailed-pair inverter, GZ34 rectifier and negative-feedback loop.

Reading against the printed chart

The drawing prints a full voltage chart, every value set at ±20 %, read to ground with an electronic voltmeter. The simulated DC operating point tracks it across the modelled stages: the reverb-driver plate lands at +414 V against a printed +410 V, the phase-inverter plates and the +77 V / +75.5 V tail fall within about a tenth of the chart (the 82 kΩ plate is the widest, +188 V against +170 V), and the 6V6 screens sit at +415 V with their grids on the −35 V bias line. The preamp rail is not held at a figure of its own: the chart prints none, so it is solved through its 10 kΩ dropper from the +325 V node, and the six preamp plates are a genuine check on the chart rather than an echo of it: they land between +176 V and +180 V against printed figures of +170 V and +180 V. The widest miss is at the two shared cathodes, boxes A and E, each +1.48 V against a printed +1.3 V — about a seventh high.

One stage is reported rather than compared:

• The tremolo oscillator. Its printed pins (+270 V plate, +2.1 V cathode) are the running average a meter reads while it swings, set by grid-leak detection; a running oscillator has no static operating point, so it is left out of the DC solution. Its plate loads return to the +415 V node after the choke, which the solution holds at its printed value, so leaving it out moves no other reading.

Every gated node verifies against the printed chart within the drawing's own ±20 % convention, with the tremolo oscillator set aside above as a documented exclusion rather than force-fitted to the chart.