AB763 Blackface Deluxe Reverb-style · 1963–1967 · 22 W

The circuit most players picture when they think "Fender clean": a 22-watt, 6V6 combo with tube reverb and a genuine tremolo, built on the AB763 chassis that ran from 1963 to 1967. 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 (each a 68 kΩ stopper on a 1 MΩ leak) → first 12AX7 stage (100 kΩ plate load, 1.5 kΩ cathode with a 25 µF bypass) → a treble-bass tone stack (250 kΩ treble and bass, 6.8 kΩ bleed, 250 pF and 0.047 µF caps) and a 1 MΩ volume → the mixing resistor into the phase inverter. No reverb, no tremolo.

Vibrato channel. Input stage as above → tone stack (with a 47 pF bright cap across the treble) and volume → a second 12AX7 stage (100 kΩ plate, 820 Ω cathode) → the reverb and tremolo section.

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 is blended back with the dry signal by the 100 kΩ Reverb control, then handed to the mix driver that feeds the inverter.

Tremolo. A 12AX7 phase-shift oscillator (Speed on a 3 MΩ control) drives an optocoupler — a neon lamp facing a photoresistor — that periodically shunts the mix-driver's grid to ground, swinging the volume up and down. The Intensity control sets how hard the lamp is driven.

Phase inverter and output. A 12AT7 long-tailed pair (82 kΩ and 100 kΩ 5% plate loads, a 470 Ω cathode resistor to a tail junction, 22 kΩ tail to ground, both 1 MΩ grid leaks returned to that junction) splits the signal for the 6V6GT pair. The output tubes are fixed-biased at −35 V through 220 kΩ leaks, with 470 Ω · 1 W screen resistors, and an 820 Ω negative-feedback loop returns from the speaker to the inverter.

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 and the 820 Ω-cathode preamp stages → a second 10 kΩ dropper → the channel-input rail. A separate negative supply — a 470 Ω · 1 W feed, a rectifier, 25 µF and 50 µF cans 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. That ancestry edge lands in the lineage graph when the 6G3 does.

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 a tenth of the chart, and the 6V6 screens sit at +415 V with their grids on the −35 V bias line. Two nodes are reported for information only:

• The two channel-input plates. Their supply rail is shared with the tremolo oscillator, a phase-shift oscillator that has no static operating point and so is left out of the DC solution. With that load absent, the rail runs above its loaded factory value and the two input plates read high — so the printed +180 V / +170 V are shown for reference 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. Solved as a quiescent stage, the 220 kΩ / 2.7 kΩ node sits near +200 V — about a quarter below the printed +270 V — so it is reported rather than fitted to the chart.

Every gated node verifies against the printed chart within the drawing's own ±20 % convention — the worst sits about a tenth off — with the tremolo oscillator and the two shared-rail input plates set aside above as documented exclusions rather than force-fitted to the chart.