Nox30

Reverse-engineered topology map for the Nox30 amp model and its reference circuit.

Nox30 is Greybound's circuit-informed amp model. Its reference target is a JMI-era AC30/6 fitted with the OS/010 Top Boost unit. There is no single canonical circuit across all production years, so Greybound treats Nox30 as a chosen engineering target rather than a universal clone.

Modeling Scope

The current model is a real-time graybox implementation. It is not a SPICE netlist and not a component-exact wave-digital simulation. The OS/010 tone network is the most circuit-derived part: it is solved with trapezoidal Modified Nodal Analysis.

Circuit Diagram

Diagram data lives in knowledge/models/amps/diagrams/nox30.diagram.json5.

This JSON5 graph is a documentation and renderer artifact. It maps each stage to the current Rust emulation and marks SPICE export as intentionally empty until subcircuits are promoted to concrete component data.

Extracted Topology

Top Boost volume:

500 kOhm volume potentiometer.
100 pF bright cap.
Modeled as frequency-dependent gain before the first Top Boost ECC83 stage.

First Top Boost triode:

ECC83 / 12AX7 common-cathode gain stage.
100 kOhm plate resistor.
1.5 kOhm cathode resistor.
25 uF cathode bypass capacitor.
Modeled as asymmetric nonlinear gain with cathode-bypass frequency response.

Tone network:

1 MOhm treble potentiometer.
1 MOhm bass potentiometer.
50 pF treble capacitor.
Two 22 nF bass-path capacitors.
100 kOhm slope path.
10 kOhm bass output/ground path.
Approximate cathode-follower source impedance: 820 Ohm.
Downstream load target: 220 kOhm.
Implemented as a seven-node passive MNA solve.

Recovery / follower behavior:

ECC83 / 12AX7 stage feeding the passive tone network.
50 kOhm cathode resistance target for the follower behavior.
Modeled as low-distortion drive into the passive network.

Phase inverter and Cut:

ECC83 / 12AX7 long-tail-pair phase inverter.
Opposed nonlinear phases with small gain imbalance.
Cut control is across the opposed phase-inverter outputs.
Cut control target: 500 kOhm pot with 4.7 nF capacitor.

Power stage:

Four EL84 tubes, two per phase.
Hot cathode-biased push-pull class AB behavior.
50 Ohm shared cathode resistor.
250 uF cathode bypass capacitor.
Plate supply target around 320 V.
Output transformer primary target around 4 kOhm.
No negative feedback in this target.
Modeled with push-pull saturation, dynamic cathode-bias shift, and supply sag.

Speaker:

The speaker IR is an optional external impulse-response stage.
It is not part of the amp core.

Nox30 Component Boundaries

Nox30 is structurally complete enough for component-level replacement work. The model exposes boundaries for:

input volume and bright-bypass behavior,
first nonlinear ECC83 common-cathode stage,
cathode follower,
Top Boost tone stack,
drive and recovery stages,
long-tail-pair phase inverter,
cut/presence shaping,
EL84 push-pull power stage,
shared B+ sag network,
output transformer filtering and core-flux state.

Observable State

VoxAmp::nox30_operating_point() exposes:

preamp, phase-inverter, and power rail voltages,
ECC83 stage currents and cathode voltages,
long-tail-pair currents and cathode voltage,
EL84 plate current, screen current, screen voltage, and cathode bias,
output transformer core flux.

This telemetry is used by monitor logging and future graybox/component training.

Validation State

Current validation is block-level and deterministic:

sample WAV render stays in fixed RMS/peak/checksum bands,
Nox30 does not go silent over long sample renders,
shared supply rails sag and recover,
EL84 screen voltage sags under sustained drive and recovers after overload,
transformer blocks DC, rolls off highs, and compresses low-end drive,
triode stages match the current ngspice fixture tolerances.

Source Notes

Reference sources used for engineering extraction include public circuit archives and service-reference sites. Do not commit their scans or PDFs. Link to public sources when needed and keep the repository content to our topology map and implementation notes.