The vocabulary this archive actually uses, defined once and linked
everywhere else. Each entry has its own address — #cathodyne,
say — so any amp page or reference note can point straight at a definition.
The one or two published datasheet operating points — a plate voltage, grid voltage, plate current, and transconductance read straight from a manufacturer's "typical operation" table — that a tube's SPICE model is fitted to reproduce exactly. Anchors come only from datasheets, never from an amplifier's own voltage chart, so a model can later verify that chart independently instead of being tuned to match it.
Appears in 12AX7 calibration study·12AX7 reference page
The standardized pin arrangement of a tube's base, identified by a short RETMA/EIA code — 9A, 7BT, 7AC, 5T, 5DA in this archive — that names one fixed wiring of elements to pins. Tubes sharing a code share a socket wiring, which is why the 6V6GT, 5881, and KT66 all read 7AC and drop into the same output-stage layout; every diagram here is read bottom view, from the wiring side.
Appears in Tube pinout diagrams·12AX7 reference page
An output tetrode design — the 6V6GT, the 5881/6L6 family, the KT66 — that uses shaped beam-forming plates instead of a true suppressor grid to focus the electron stream and suppress secondary emission, giving higher plate efficiency than a plain tetrode of similar size. Every power tube modeled in this archive is a beam-power type, and all three share the 7AC octal basing.
Appears in 6V6GT reference page·KT66 reference page
The DC offset that holds a tube's control grid negative relative to its cathode, setting where on the tube's curve it idles. Cathode bias lets a stage set its own operating point off the voltage a resistor develops between cathode and ground; fixed bias instead applies a separate negative supply straight to the grid, which is how every push-pull output stage in this archive is biased.
Appears in 5F6-A — Tweed Bassman-style·5E1 — Tweed Champ-style
The rectified, filtered high-voltage DC supply that feeds tube plates and screens — the name survives from multi-battery radio sets, where "A" was the filament battery, "B" the plate battery, and "C" the grid-bias battery. It starts at several hundred volts at the rectifier and steps down through a choke or dropping resistors to each stage after it, losing a little rail at every step.
Appears in 5E1 — Tweed Champ-style·5F6-A — Tweed Bassman-style
A small capacitor, often around 100 pF, wired across a channel's volume potentiometer so a treble-boosting bypass path opens as the wiper turns down — the cap matters least at full volume and most in the lower range. Several circuits here fit it only to the bright-marked input or channel, which is where the name comes from.
Appears in Capacitor code chart & decoder·JTM45 — British lead-style
A capacitor placed across a cathode resistor so signal current can skip around it to ground while DC bias current still passes through; without it, the cathode resistor also degenerates (reduces) the stage's AC gain. Leaving one triode of a shared-cathode pair deliberately unbypassed, so a negative-feedback signal has somewhere to act, is a recurring trick in the smaller circuits here.
Appears in 5F10 — Tweed Harvard-style·5E1 — Tweed Champ-style
A triode stage whose output is taken from the cathode instead of the plate, giving voltage gain just under unity, very low output impedance, and no phase inversion. Several circuits here run one direct-coupled straight out of a gain stage to drive the tone stack, buffering it from the stages before and after.
Appears in 5F6-A — Tweed Bassman-style·JTM45 — British lead-style
A phase-inverter topology built from a single triode with matched plate and cathode load resistors, so one input produces two output signals of equal amplitude and opposite phase — also called a split-load or concertina inverter. It is the phase inverter this archive's smaller push-pull circuit uses in place of a second gain tube.
Appears in 5F10 — Tweed Harvard-style
An iron-core inductor wired in series inside the B+ supply, between filter capacitors, that smooths ripple more effectively — and with a softer voltage sag under load — than a plain dropping resistor would in its place. One circuit here traded its choke for a resistor in the very next design revision, giving up some of that sag for one fewer iron part to buy.
Appears in 5E1 — Tweed Champ-style·5F6-A — Tweed Bassman-style
A capacitor wired in series between two stages to pass AC signal while blocking the DC plate voltage of the stage before it. Paired with the following stage's grid-leak resistor it forms a high-pass filter, and that pair's corner frequency sets how much bass survives into the next stage.
Appears in Amp builder calculators·5E1 — Tweed Champ-style
A small capacitor — typically 0.05 µF at 600 V — wired from one leg of the AC line to the chassis on amps built for two-wire, ungrounded mains, giving the chassis a high-impedance path toward the supposed neutral to shunt hum. With a reversed plug, a ground switch on the hot leg, or the cap failed shorted, it puts full line voltage on the chassis — the fault that earned the name — so modern practice removes it and fits a three-wire grounded cord.
Appears in High-voltage safety — the death cap
Shorthand — Fender / Marshall / Vox — for the three-knob passive treble-bass-mid tone-stack topology all three makers built around: a treble cap and bass cap in a bridged network, tapped by pots, with the mid control returning the network's floor to ground through a fixed slope resistor. It sits after the gain stages and ahead of the phase inverter, and it always costs some signal level on the way through.
Appears in 5F6-A — Tweed Bassman-style·JTM45 — British lead-style
A resistor — 1 MΩ is the common value in this archive — that returns a triode's control grid to ground, or to a phase-inverter tail junction, giving a defined DC path for any coupling-cap leakage rather than letting the grid float. Paired with the coupling cap ahead of it, its value also sets that stage's bass corner frequency.
Appears in 5E1 — Tweed Champ-style·Amp builder calculators
A small resistor, typically tens of kΩ, wired directly at a control grid right at the tube socket to damp parasitic RF oscillation and cushion the grid against an input signal swinging positive. Every input jack in this archive carries one, and output-tube grids often get a second, larger-value stopper of their own.
Appears in JTM45 — British lead-style·5E1 — Tweed Champ-style
The resistive filament element that heats a tube's cathode so thermionic emission can occur — or, in a directly-heated rectifier, that filament is the cathode itself. Heater wiring is universally AC and never affects a tube's DC operating point, which is why it is omitted from the redrawn schematics in this archive.
Appears in Tube pinout diagrams·5Y3GT reference page
The ancestry this archive records between circuits: a direct "derived from" edge for a traceable descendant redrawn stage-for-stage from an earlier design, and a looser "influenced" edge where ideas spread without a literal copy. Lineage is metadata, cited like any other fact — never a stand-in for the maker names this project keeps out of ids and titles.
Appears in JTM45 — British lead-style·5F6-A — Tweed Bassman-style
A phase-inverter topology using two triodes sharing a common "tail" — a cathode resistor or network returned toward ground or a negative supply — so a signal into one grid appears inverted and amplified equally at both plates. It is the phase inverter behind every push-pull circuit in this archive larger than the smallest amps.
Appears in 5F6-A — Tweed Bassman-style·JTM45 — British lead-style
The three figures a datasheet's "typical operation" table reports for a triode at a stated point: µ (amplification factor) is how many volts of plate swing one grid volt is worth; gm (transconductance, µmho) is how much plate current changes per grid volt; rp (plate resistance, kΩ) is the tube's own internal AC resistance. The three are never independent — µ = gm × rp always holds — which is one of the cross-checks this archive's calibration study used to validate a contested datasheet reading.
Appears in 12AX7 calibration study·12AX7 reference page
Negative feedback: a fraction of the output transformer's secondary signal fed back, out of phase, into an early stage — usually the phase-inverter tail — to trade some gain for lower output impedance, wider frequency response, and lower distortion. The presence control works from inside this loop.
Appears in 5F6-A — Tweed Bassman-style·5E1 — Tweed Champ-style
The specific plate voltage, grid voltage, and resulting plate current at which a tube sits at idle — what a factory voltage chart records node by node, and what this archive's simulation is checked against. A stage's operating point is set by its plate load, its bias arrangement, and the B+ rail feeding it.
Appears in 12AX7 calibration study·5F4 — Tweed Super-style
The output transformer (OT) couples the output tubes to the low-impedance speaker load, its turns ratio set for the plate-to-plate load those tubes want; the power transformer (PT) steps the mains up to the high-tension winding that feeds the rectifier, and down to filament voltage. Both are cited by their manufacturer part number wherever the source schematic prints one.
Appears in 5F6-A — Tweed Bassman-style·Amp builder calculators
A stage that turns one signal into two copies of equal amplitude and opposite phase to drive a push-pull output pair. This archive models two families of it — the single-tube cathodyne and the two-tube long-tailed pair — and a single-ended amp needs neither.
Appears in 5F10 — Tweed Harvard-style·5F6-A — Tweed Bassman-style
The resistor — or, in an output stage, the transformer primary — between a tube's plate and B+, across which the tube's varying plate current develops the stage's output swing. Its value trades gain against headroom, and against how far it lets the B+ rail sag into the next stage down the chain.
Appears in Amp builder calculators·12AX7 reference page
A control, usually a pot inside the negative-feedback loop, that shelves how much high frequency reaches the phase-inverter tail. Because feedback normally suppresses gain, removing treble from the feedback path makes an amp sound brighter and more open as the control is turned up, rather than boosting treble directly.
Appears in 5F6-A — Tweed Bassman-style·JTM45 — British lead-style
An output stage using two or more tubes that alternately handle the positive and negative halves of the waveform, driven out of phase by a phase inverter and recombined across the output-transformer primary. It cancels even-order distortion and supply ripple that a single-ended stage cannot, and is how every output stage in this archive above the smallest amps is built.
Appears in 5F6-A — Tweed Bassman-style·6V6GT reference page
The stage that turns the power transformer's AC high-tension winding into the pulsating DC that filter capacitors and a choke then smooth into B+. This archive covers tube rectifiers (full-wave from a center-tapped winding) and solid-state diode bridges; a tube rectifier's own forward drop, and how it rises under load, is a deliberate part of a vintage amp's sag.
Appears in 5Y3GT reference page·GZ34 reference page
The first filter capacitor after the rectifier, charged near the peak of the rectified waveform and partly discharged between peaks by the current the circuit draws. Its size, together with the rectifier's own characteristics, sets how much the B+ rail sags and ripples before a choke or resistor smooths it further downstream.
Appears in 5E1 — Tweed Champ-style·JTM45 — British lead-style
The momentary drop in B+ voltage under playing dynamics, most pronounced with a tube rectifier — whose forward drop rises with the current drawn through it — feeding a modest reservoir capacitor. Heard as a soft compression and "bloom" on transients, it is a deliberately kept character of this archive's smaller circuits rather than a fault to design out.
Appears in 5F10 — Tweed Harvard-style·5Y3GT reference page
The second grid in a tetrode or beam-power tube, held near B+ between the control grid and the plate, where it accelerates electrons toward the plate while shielding the control grid from the plate's field — giving far more gain than a triode of similar size. It draws its own screen current and is usually fed through its own dropping resistor, separate from the plate supply.
Appears in Tube pinout diagrams·6V6GT reference page
A solid-state metal-oxide rectifier technology common from the 1940s into the 1960s, before silicon diodes took over, used in this archive's bias supplies to turn a small AC tap into the fixed negative grid voltage that biases an output stage. Selenium rectifiers age by rising forward drop, and a failing one is notorious for an acrid smell.
Appears in 5F6-A — Tweed Bassman-style·5F4 — Tweed Super-style
An output stage built around one power tube, passing the whole signal waveform through it rather than splitting the load between a push-pull pair. It needs no phase inverter, runs class A, and its even-order distortion and supply-ripple sensitivity are part of the compressed voice of the smallest amps in this archive.
Appears in 5E1 — Tweed Champ-style·6V6GT reference page
A switch placed between the rectifier's output and the rest of the B+ supply, letting the heaters stay warm while high voltage is held off the plates. It lets a player silence an amp between sets without a full thermal cycle, and spares a tube rectifier from delivering full B+ into cold, high-impedance tubes. Present on every circuit in this archive, it is omitted from the redrawn schematics since it never affects DC operating point.
Appears in JTM45 — British lead-style·5F4 — Tweed Super-style
The allowed spread between a stated value and what an actual part, or measurement, may show while still being in spec. A component's tolerance is a printed percentage — a resistor's color band, a capacitor's letter code — while each factory voltage chart modeled in this archive carries its own stated measurement tolerance, most often ±20%, against which this archive's simulated-versus-chart verification is judged.
Appears in Resistor color code calculator·12AX7 calibration study·Units & notation guide
The passive network of capacitors, resistors, and usually three potentiometers — bass, mid, and treble — that shapes an amp's frequency response between the gain stages and the phase inverter. Being entirely passive, it always costs some signal level, which is why several circuits here buffer it with a cathode follower.
Appears in 5F6-A — Tweed Bassman-style·Amp builder calculators
The two status values this archive assigns a circuit's metadata. Every new circuit lands as draft; verified is set only once its simulated DC operating point falls within the published chart's own tolerance and a maintainer signs off — CI can propose the check, but only a maintainer can promote the badge.
Appears in 5F6-A — Tweed Bassman-style (verified)·5F2-A — Tweed Princeton-style (draft)
Definitions are written for this archive, in plain language, and
checked against the datasheets and design texts in the reference
library — they describe usage here, not a formal standard. Found a term used on a
page that isn't listed? Open an issue or a
pull
request against reference/glossary.yaml.