Slightly Technical

Tools · Free

Feedback Lab

Turn a presence knob and watch a real feedback loop open up. 29 amps (24 schematic-verified), their negative-feedback circuits traced from factory schematics and simulated as closed loops — into real, reactive speaker loads.

Every tone-stack tool stops at the phase inverter. The power amp behind it is not a wire: it is a feedback loop wrapped around the output transformer and the speaker, and the presence control lives inside that loop. This simulates the loop itself — gain blocks, transformer, feedback divider, reactive cab — so you can see the response at the speaker, the feedback being applied at every frequency, and the damping it buys.

The folklore runs deep here. "A JTM45 is just a Bassman" — same feedback resistor, same presence pot, twice the feedback, because of which transformer tap the wire comes off. "Blackface Fenders have no feedback character" — a Twin runs more feedback than almost anything on this page; it just has no knob on it. The numbers under each amp below are computed by the simulator from the traced circuits, not quoted from forums.

The library

29 amps across 6 lineages

24 of them have a feedback network traced straight from a factory schematic; the rest are single-source traces or documented estimates, marked as such inside the tool. The dB figure is the midband feedback each amp runs (presence at minimum, documented takeoff tap), computed live by the same engine that draws the curves. Pick an amp to open it in the lab.

Fender lineage

From tweed presence to the presence-less blackface loop

Marshall lineage

27k@16Ω → 47k@8Ω → 100k@4Ω: the feedback retreat, era by era

American high-gain

From tweed presence to the presence-less blackface loop

European high-gain

From tweed presence to the presence-less blackface loop

Boutique high-gain

From tweed presence to the presence-less blackface loop

British lineage

From tweed presence to the presence-less blackface loop

Honest numbers

What is exact and what is estimated

The feedback networks — resistors, presence pots, caps, takeoff taps — are read from factory drawings for 24 of the 29 amps, cited per amp inside the tool. The rest are a single-source trace, an inference off a low-resolution scan, or — one amp — no surviving schematic at all, each flagged as such rather than presented as equally solid. The gain blocks and output-transformer impedances are documented estimates (tube curves and measured lore, good to roughly ±1.5 dB of feedback), and every one of them is editable under the hood. The curve shapes come from the network, which is exact for the traced amps; the absolute dB figures are honest engineering estimates throughout, labelled as such.

Reading the views

Three things the loop tells you

Presence is removal, not boost

A presence control never adds anything. It shorts out the feedback divider at high frequencies, so the loop stops correcting there and the response rises toward what the amp does with no feedback at all. Switch to the Feedback view and turn the knob: the curve that moves is the amount of correction being thrown away.

The famous numbers are takeoff taps, not magic

A JTM45 uses the Bassman's exact 27k resistor and 5k presence pot — and runs roughly eight decibels more feedback, because it picks the signal off the 16Ω tap where the Bassman uses a 2Ω winding. Marshall then spent a decade removing feedback: 15 dB, 12 dB, 6 dB across the eras. That progression is most of what 'vintage Marshall feel' means, and you can A/B every step of it.

The speaker is inside the loop

A real cab's impedance triples at its bass resonance and rises again in the treble, and both swings pass through the feedback loop. That is why the same amp feels different into different cabs — and why the Damping view, which is what 'tight' and 'loose' actually measure, changes when you swap the cab model. No passive tone-stack tool can show this.

Also on the bench

The other half of the same amp

Feedback Lab starts where the tone stack hands over to the phase inverter. Tone Stack Lab covers everything before that moment — the treble, mid and bass controls of 55 amps and pedals, solved from the same kind of factory-schematic tracing.

Open Tone Stack Lab →

Free, and staying that way

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Modelled the loop and the amp still sounds wrong on the bench? I take a small number of one-to-one consultations →