AA964 Blackface Princeton‑style · 1964–1968 · 12 W

For fifteen years the Princeton was a Champ with a tone control: one small bottle driving a single 6V6. The AA964 is what the name meant after that idea was retired — a fixed-bias push-pull pair of 6V6GTs at around 12 watts, a treble-and-bass tone stack, and a tremolo that works by wobbling the output tubes' own bias. Anyone reading the tweed 5F2-A's single-ended voltages onto a blackface Princeton is reading the wrong amp; the two share a name and almost nothing else.

Only two small bottles do the small-signal work. A 7025 — the low-noise 12AX7 the drawing calls for — carries both gain stages. A single 12AX7 is split down the middle: one triode runs the tremolo oscillator, the other is the whole phase inverter.

Signal path

Two input jacks, each a 68 kΩ stopper into the grid, with the 1 MΩ leak at input 1's tip → V1A (100 kΩ plate load, 1.5 kΩ cathode with a 25 µF bypass) → a treble/bass tone stack (250 kΩ treble and bass controls, a 100 kΩ slope resistor, 6.8 kΩ bleed, and 250 pF · 0.1 µF · 0.047 µF caps) and a 1 MΩ volume → V1B, an identical 100 kΩ / 1.5 kΩ stage → 0.022 µF into the inverter.

Phase inverter. One triode does it: a cathodyne, 56 kΩ above the plate and 1 kΩ below the cathode into a 56 kΩ tail to ground, with the 1 MΩ grid leak returned to that junction so the grid sits about a volt under the cathode. The plate and the cathode each hand a 0.1 µF coupling cap to one output tube. It provides no voltage gain — which is why V1B, the stage in front of it, has to be a full gain stage rather than a mixer.

Output. The 6V6GT pair is fixed-biased at −34 V through 220 kΩ · 5 % grid leaks. The screens tie straight to the +415 V rail with no stopper resistors, and the plates work into the 125A10B transformer. One global feedback loop closes the amp: 2.7 kΩ from the speaker back to a 47 Ω tail sitting underneath V1B's cathode resistor. That 47 Ω is the whole trick — V1B's 1.5 kΩ is bypassed, so the returning signal has an unbypassed 47 Ω to develop across and nothing else.

Tremolo on the bias line

Half the 12AX7 runs a three-section phase-shift oscillator — 0.02 µF, 0.01 µF and 0.01 µF against a 3 MΩ Speed control and two 1 MΩ resistors, with a 220 kΩ plate load and a 3.3 kΩ bypassed cathode. What it does with its output is the part worth knowing.

Bigger blackface amps spend a whole optocoupler on tremolo: a neon lamp and a photoresistor that shunt a preamp grid to ground. The Princeton has no triode to spare and no lamp. Instead the oscillator's output runs through 1 MΩ and a 0.1 µF cap into a 250 kΩ Intensity control whose wiper is the −34 V bias line. The oscillator therefore pushes the output pair's grid bias up and down directly, swinging the idle current of the whole output stage rather than modulating a small-signal grid. A footswitch jack lands on the phase-shift ladder and shorts it to ground to silence the oscillator.

At DC none of this matters: the oscillator reaches the Intensity control through a blocking capacitor and the grid leaks draw essentially nothing, so no current flows in the control and the bias line sits at the supply's −34 V wherever the knob is set.

Power

TR1 (125P1B), 340-0-340 V → GZ34 → +420 V reservoir, feeding the output transformer's centre tap and both 6V6 plates → 1 kΩ · 1 W → +415 V screens → 18 kΩ · 1 W → a filtered intermediate node → 18 kΩ · 1 W → +290 V for both 7025 plates and the inverter. Four 20 µF · 450 V cans do the filtering. The negative supply is as plain as it gets: 100 kΩ from the high-voltage winding, a silicon diode, a 25 µF · 50 V can with a 27 kΩ bleeder, and out through the Intensity control at −34 V.

The heater circuit: a floating pair about a grounded centre tap

The 6.3 V winding is centre-tapped, and the layout drawing grounds it at the tap: the transformer's green-yellow lead meets the red-yellow HT centre tap at a solder point and both go to a ground arrow, while the two green leads run up to the pilot light, one to each of its terminals. From each lamp terminal an arrow says "to all 6.3 volt heaters" — the chain is arrowed, not routed — so the socket order on the board here is a convention, and only the landings are the sheet's. Those were read at the drawing's native 777 ppi with each socket's circle fitted to its drawn rim and the pin bearings measured: at the 7025 and the 12AX7 a short bow outside the rim ties pins 4 and 5 into one leg and pin 9 is a bare terminal, the 6.3 V parallel arrangement; at both 6V6GT sockets pins 2 and 7 are bare terminals, the two legs the arrow stands in for. Neither leg is grounded, so which is called feed and which return is only a name.

Two earlier readings of this sheet did not survive the measurement. A stroke that seemed to join the 7025's pins 8 and 9 at 9× on a 2166 px copy is the socket rim: once the circle is fitted, the ink between those pins has the same inner and outer edge as every other arc of the same rim and nothing lies outside it. And the board used to draw this supply as a twisted pair whose hops alternated pins socket to socket — the 12.6 V series picture on a 6.3 V amplifier, and unprovable besides, because a pair drawn that way puts one socket's return on the next socket's feed. It is drawn now as two green conductors, one per leg, with the lamp across them at the transformer end.

The tone network, as the drawings wire it

The published AA964 schematic and its factory layout sheet agree: the 250 pF treble capacitor and the 100 kΩ slope resistor both leave the first triode's plate; 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 bass pot is a rheostat (the drawing's arrow-through-body variable resistor) in series down to the 6.8 kΩ bleed, whose top also takes the 0.047 µF from the slope foot; and the stack's output is the treble pot's wiper alone, straight into the volume pot. The textbook redrawing of these parts puts the treble pot's cold end on the slope foot and joins the treble and bass wipers at one output node; the schematic, the board diagram and the tone-stack lab all follow the sheets.

Reading against the printed chart

The drawing prints a full voltage chart, every value at ±20 %, read to ground with an electronic voltmeter. Only the +420 V reservoir is supplied to the simulation; the screen rail and the preamp rail are solved through the drawing's own droppers, so the printed +415 V and +290 V are results rather than inputs. Both land close — the screen rail lands at +412 V against the printed +415 V, the preamp rail about 2 % high — and every tube pin follows: both 7025 plates near +195 V against a printed +190 V, their cathodes at +1.54 V and +1.56 V against a printed +1.5 V, and the cathodyne at +232 V plate, +66 V cathode, +65 V junction against a printed +220 / +65 / +63.8 V. The worst gated node is the inverter plate, about 5 % off.

One node is reported rather than compared, because neither sheet prints a value for it: the 47 Ω feedback tail under V1B. The filter node between the two 18 kΩ droppers carries no value on the schematic, but the layout sheet labels that can section +370 V, so it is compared like the other rails — and lands at +354 V, about 4 % low.

The tremolo oscillator is excluded from the DC solution. Its printed pins — +260 V at the plate, +2.4 V at the cathode — are the running averages a meter reads while the circuit swings, set by grid-leak detection rather than by a static operating point. Solving it as a quiescent stage would answer a question the chart is not asking. Leaving it out costs the rest of the solve almost nothing: its plate load returns to the screen rail, so the ~0.7 mA it would draw would move that rail by well under a volt.

What sits off the eyelet board

Four 20 µF · 450 V sections in one chassis-mounted can carry the whole supply, and the last 18 kΩ dropper — the one between +370 V and +290 V — is mounted at the can rather than on the board. Only the 1 kΩ and the first 18 kΩ sit on the board. The negative supply has its own small terminal strip: 100 kΩ from the high-voltage winding, a silicon diode, a 25 µF · 50 V can and a 27 kΩ bleeder, out through the Intensity control. The 68 kΩ input stoppers and the 1 MΩ grid leak mount at the input jacks, and the tone, volume, Speed and Intensity networks mount at their pots. The board layout draws each of these where it actually lives.

This circuit is published as verified. Everything the entry claims is on the page — the component values, the netlist they came from, the comparison against the factory chart and the parts list — and the comparison reads clean: the worst gated node is 5.5 % out and the rails sit inside 2.4 %. Both drawings are here too: a redrawn schematic, and a board layout whose point-to-point wiring is machine-checked electrically equivalent to the netlist. What that badge does not claim is spelled out on the About page: it means simulation agrees with the published chart, not that anyone measured this amplifier.