AA1164 Blackface Princeton Reverb‑style · 1964–1967 · 12 W

The reverb-equipped blackface Princeton: a 12-watt, single-channel amp with tube reverb and a genuine tremolo, running a fixed-biased pair of 6V6GTs behind a split-load phase inverter. It was introduced in 1964 alongside the reverbless AA964 and carried blackface cosmetics through 1967. Six controls — Volume, Treble, Bass, Reverb, Speed, Intensity — and a 5U4GB rectifier rather than the GZ34 of its larger siblings. The preamp bottle is marked 7025 on the drawing, the low-noise selected version of the 12AX7.

Signal path

Preamp. Two inputs, each through its own 68 kΩ stopper onto a shared 1 MΩ grid 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, a 100 kΩ slope resistor, 6.8 kΩ bleed, 250 pF, 0.1 µF and 0.047 µF caps) and a 1 MΩ Volume → a second identical 12AX7 stage. There is only one channel; everything the amp does happens in this one chain.

The tone stack, as the sheet wires it. The AA1164 draws the same two-knob ladder as the AA964 it descends from — not the textbook redrawing of the same parts — and the schematic here follows the sheet (re-read at lug level 2026-08-03): the 250 pF and the 100 kΩ slope both leave the plate node; 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 above the 0.047 µF/6.8 kΩ foot (the factory layout mounts the 6.8 kΩ right on the bass pot, its far lead grounded to the case); and the stack's output is the treble pot's wiper alone, into the Volume control. The tone-stack lab plots this circuit with that wiring.

Reverb. After the second preamp stage the signal splits. A 500 pF cap taps the dry signal off to a 12AT7 with both triodes in parallel (2.2 kΩ shared cathode) that swings the 125A20B transformer and the spring tank. The return comes back through a 12AX7 recovery stage (220 kΩ grid resistor, 100 kΩ plate, 1.5 kΩ cathode) and the 100 kΩ Reverb control. Dry and wet are then summed by a pair of resistors straight onto the next grid: 3.3 MΩ for the dry path (with a 10 pF cap across it to keep the top end) and 470 kΩ for the reverb.

Driver. The mixed signal drives a third 12AX7 stage with a 100 kΩ plate load and a two-part cathode: 1.5 kΩ bypassed by 25 µF, sitting on an unbypassed 47 Ω. That 47 Ω is where the negative feedback lands — a 2.7 kΩ resistor back from the speaker — so the whole loop closes on a cathode rather than on the inverter.

Tremolo. A 12AX7 phase-shift oscillator (Speed on a 3 MΩ control) does not touch the audio path at all. Its output is coupled through the 250 kΩ Intensity control directly onto the −34 V bias line feeding the output tubes' grid leaks, so the tremolo works by rocking the output stage's bias up and down. That is a different mechanism from the optocoupler tremolo of the larger blackface amps, which shunts a preamp grid to ground instead.

Phase inverter and output. A single 12AX7 triode wired as a split-load (cathodyne) inverter: 56 kΩ in the plate, 1 kΩ plus 56 kΩ in the cathode, and a 1 MΩ grid leak returned to the junction of those two cathode resistors, which is what sets the stage's bias. Plate and cathode feed the two 6V6GT grids through 0.1 µF caps. The output tubes are fixed-biased at −34 V through 220 kΩ 5% leaks; their screens tie straight to the supply with no screen resistors, and the 125A10B output transformer drives the speaker and a parallel external-speaker jack.

Power

340-0-340 V (power transformer 125P1B) → 5U4GB → +420 V at the reservoir, which is also the output transformer's centre tap → a 1 kΩ · 1 W dropper → +400 V for the 6V6 screens and the reverb driver → 18 kΩ · 1 W → +320 V → 18 kΩ · 1 W → +240 V for every preamp plate load and the phase inverter. The 6V6 plates read +410 V, ten volts under the centre tap they hang from. A separate negative supply — a 100 kΩ 5% feed off the HT winding, a silicon rectifier, a 25 µF can and a 22 kΩ bleeder — provides the −34 V fixed bias.

Nothing taps the +320 V node except its own filter capacitor, which is why the two 18 kΩ droppers show identical 80 V drops on the printed chart.

What sets it apart

Compared with the reverb-and-tremolo blackface amps above it, the AA1164 makes three different choices.

The phase inverter is a split-load stage rather than a long-tailed pair. A cathodyne has no voltage gain of its own, so all the drive for the output tubes has to come out of the preamp ahead of it, and the two 6V6 grids are fed from one triode instead of two.

The tremolo modulates the output stage's bias instead of shunting a preamp grid through an optocoupler. Its depth therefore depends on where the output tubes are biased rather than on how much signal is at a grid.

The feedback returns to a driver cathode over a 47 Ω resistor rather than to the phase inverter. With only 47 Ω of unbypassed cathode resistance to work against, the loop is a shallow one.

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 simulation takes only the +420 V reservoir as given and solves the 1 kΩ and both 18 kΩ droppers, so the +400 V screen rail, the +320 V node and the +240 V preamp rail are results rather than assumptions — and they land 1.9 %, 2.4 % and 3.2 % from the chart. Below them the preamp plates sit at +161 V against a printed +160 V, the reverb-driver cathode at +8.0 V exactly on the chart, and the split-load inverter's three nodes no more than 12 % from their printed +200 V / +50 V / +49 V.

One node is set aside. The tremolo oscillator shares a bottle with the phase inverter, and its printed pins (+260 V plate, +2.4 V cathode) are the running average a meter reads while it swings, set by grid-leak detection rather than by a static operating point. It is reported rather than fitted to the chart. Its plate load draws about half a milliamp from the screen rail, so leaving it out accounts for roughly half a volt of the margin by which the three supply nodes read high.

Every gated node verifies against the printed chart within the drawing's own ±20 % convention — the worst 11.5 % off — with the tremolo oscillator set aside above as a documented exclusion rather than force-fitted to the chart.