AA764 Blackface Champ‑style · 1964–1968 · 5 W

The student amp that outlived every fashion: five watts, one 12AX7, one single-ended 6V6GT, a 5Y3GT rectifier, and an 8-inch speaker in a small black cabinet. The AA764 is what the tweed 5F1 became when Fender restyled the line in 1964 — the same two-stage preamp into the same cathode-biased single-ended output stage, now with a treble and bass tone stack, a stiffer power supply, and a gentler negative-feedback loop. It ran through the blackface years and carried on under silverface cosmetics after 1968 with the output stage untouched.

Signal path

Two input jacks (high and low) each sit behind a 68 kΩ grid stopper and share a 1 MΩ leak to ground. From there:

V1A — 12AX7, 100 kΩ plate load, 1.5 kΩ cathode with a 25 µF bypass. Its plate feeds the tone stack directly; both stack legs start with a capacitor, so no DC reaches the controls.

Tone stack — the blackface treble-bass network. A 250 pF cap carries the top end onto the 250 kΩ Treble control; a 100 kΩ slope resistor feeds a 0.1 µF cap into the junction below the treble pot and a 0.047 µF cap into the 250 kΩ Bass control's lower leg, which returns to ground through 15 kΩ. The treble wiper hands the recovered signal to a 1 MΩ Volume control.

V1B — 12AX7, 100 kΩ plate load. Its cathode resistor is 1.5 kΩ with a 25 µF bypass, but it does not land on ground: it sits on a 47 Ω leg, and the 2.7 kΩ negative-feedback resistor from the speaker jack lands on that same junction. The feedback voltage develops across the 47 Ω and appears at the cathode, which is why the bypass can spans only the 1.5 kΩ.

Output — a 0.02 µF coupling cap into the 6V6GT grid, held down by a 220 kΩ leak. The 6V6 is cathode-biased on 470 Ω · 1 W with a 25 µF bypass, its screen wired straight to the second filter node with no stopper, and its plate working into the 125A35A single-ended output transformer.

Power

320-0-320 V from the 125P1B power transformer → 5Y3GT full-wave rectifier → three 20 µF · 450 V filter cans. The chain reads +360 V at the reservoir (which also feeds the output transformer primary), +350 V after a 1 kΩ · 1 W dropper at the 6V6 screen, and +330 V after a 10 kΩ · 1 W dropper at both 12AX7 plate loads. A 1 A slow-blow fuse and an AC switch sit ahead of the primary.

The 6.3 V heater winding is single-ended: one of its two green leads is grounded at the chassis and the other feeds the pilot lamp and both heaters. Each socket therefore takes one green feed and returns through chassis ground — pin 2 fed and pin 7 grounded on the 6V6GT, pins 4 and 5 strapped together and fed with the centre tap at pin 9 grounded on the 12AX7. The 5Y3GT is directly heated from the separate 5 V winding and sits outside that chain.

On the board

The eyelet board runs the length of the chassis, power end at the left. The two rail droppers sit in a three-node stack over the filter sections they split; then the 6V6's 470 Ω · 1 W cathode resistor with its 25 µF can, the 2.7 kΩ feedback resistor, the 1.5 kΩ/25 µF driver cathode pair and the 47 Ω divider leg, the 220 kΩ grid leak sharing an eyelet with the 0.02 µF coupler, and the two 100 kΩ plate loads meeting at a single +330 V tie point. The tone capacitors follow — 250 pF, 0.047 µF, 0.1 µF — beside the 100 kΩ slope resistor, and the board finishes with the two 68 kΩ input stoppers and the first stage's 1.5 kΩ/25 µF cathode pair. Two parts are chassis-mounted rather than on the board: the 1 MΩ grid leak at input jack 1 and the 15 kΩ bass leg at the Bass control.

What changed from the 5F1

Three things, all visible on the chart:

The stiffer supply matters most. In the tweed circuit the 6V6 screen sat 45 V below its plate; here it sits at the same +350 V the plate does, and the preamp plates run at +330 V rather than +250 V. The result is a louder, firmer, later-breaking amp than the tweed Champ, with the tone stack giving back some of the top end the extra headroom exposes.

Reading against the printed chart

The drawing prints a full voltage chart, every value set at ±20 %, read to ground with an electronic voltmeter. Driving the reservoir at its printed +360 V and solving everything below it:

Every node lands inside the drawing's own ±20 % convention, but the 6V6 cathode lands close to the edge of it, and the reason is worth stating plainly: the tube models here are fitted to the datasheet's 250 V operating point, and this amp runs its 6V6 with both plate and screen near 350 V — far enough outside the anchor region that the model draws richer current than a real bottle does. The preamp plates read high for the mirror-image reason the tweed Champ's read high: the models are datasheet-typical while Fender measured 1964 production tubes.

One value on the chart is deliberately not simulated. The drawing prints +360 V at the reservoir and +350 V at the 6V6 plate; the 10 V between them is the output transformer primary's winding resistance, which the drawing does not publish. Rather than invent a figure to close the gap, the primary resistance is left out and the plate is treated as the reservoir node.

The board-layout diagram's point-to-point wiring is machine-checked against this same simulated netlist: every modelled part and every socket pin is proved to sit on the node the circuit puts it on. The heater chain, the pilot lamp and the transformer/rectifier AC side sit outside that check and are drawn as an annotation layer.