Vacuum tube audio glossary

Plain-English definitions of 42 terms used across the Amperatubes catalog, calculators and learning guides. Each entry links to a dedicated page with cross-references and the contexts in which the term shows up on the site.

Anatomy (7)

Cathode: Electron source. Heated directly (filament IS the cathode — 300B, 2A3) or indirectly (separate heater warms an oxide-coated sleeve — 12AX7, 6SN7). Emits electrons by thermionic emi…
Anode (Plate): Positive electrode that collects the electrons emitted by the cathode. Held at high positive DC potential (B+). Material and shape (carbon-coated nickel, ribbed grey plate, black p…
Control grid (g1): Wire mesh between cathode and anode. The signal voltage applied to g1 modulates the much larger anode current — this is the source of amplification. Typically biased negative relat…
Screen grid (g2): Second grid in tetrodes and pentodes, held at a positive DC voltage. Reduces grid-plate capacitance (Miller), boosts gain, and stabilises plate current. Usually decoupled with a la…
Suppressor grid (g3): Third grid in pentodes, tied to cathode. Repels secondary electrons emitted from the plate back to the plate, preventing tetrode 'kink' (negative-resistance region). Beam tetrodes…
Heater: AC or DC element that raises the cathode to ~750°C for thermionic emission. Indirectly heated tubes use a separate heater inside the cathode sleeve; directly heated tubes (DHT) use…
Getter: Reactive metal (usually barium) flashed on the inside of the envelope after evacuation. Absorbs residual gas and any later in-leakage. The silver or grey mirror is visual proof the…

Architecture (4)

Beam tetrode: Tetrode variant (RCA 1936) using beam-forming plates instead of a suppressor grid to focus electrons and prevent secondary emission. Examples: 6L6, KT66, KT88, 6V6, 5881. Combines…
DHT: Directly heated triode: the heater filament is also the cathode. Examples: 300B, 2A3, 45, 211, 845, PX25, PX4. Revered for tonal purity (no cathode sleeve to filter the signal) but…
IDHT: Indirectly heated triode/pentode: separate heater inside an oxide-coated cathode sleeve. Better hum rejection than DHT, faster warm-up uniformity, easier cathode-bias circuits. Exa…
Compactron: GE 1961 12-pin miniature base used to integrate multiple sections (triode-pentode, triode-triode-pentode) for late-tube TV chassis. Examples: 6JC6, 6LB6, 6KD6. Cheaper than separat…

Biasing (3)

Cathode bias (self-bias): Bias produced by a resistor in the cathode return: Ia × Rk = grid-cathode bias voltage. Self-correcting (more current → more bias → less current). Loses some headroom but extremely…
Fixed bias: External negative DC supply applied to the grid. No cathode resistor needed, full headroom available, but no thermal self-correction — bias must be set per tube and re-checked when…
Grid leak bias: High-impedance resistor (1-10 MΩ) from grid to cathode. Bias is generated by tiny grid current charging the coupling capacitor through the leak resistor. Common in low-power preamp…

Components (1)

Compactor cap: Capacitor that passes audio signal between stages while blocking the DC plate voltage. Value sets the low-frequency cutoff (-3 dB at f = 1/(2π × R × C) where R is the next grid lea…

Market (2)

NOS: New Old Stock: factory-fresh tube manufactured during the original production era (typically 1940s-1980s) and never used. Distinct from 'used-tested' (NIB testing only) and 'reissu…
Equivalents: A tube intended to be electrically and mechanically interchangeable with another. Some are direct (12AX7 ≡ ECC83 ≡ 7025), some need resocketing or rebiasing (5751 ≈ 12AX7 lower mu)…

Operating Class (2)

Class A: The tube conducts for the entire signal cycle (360°). Lowest distortion, lowest efficiency (≤25% SE, ≤50% PP). Bias placed near the centre of the loadline. Always in the linear reg…
Class AB: The tube conducts for slightly more than 180° but less than 360°. Used in push-pull pairs. Higher efficiency than A, with crossover region designed to overlap so distortion stays l…

Operation (2)

Q-point (operating point): The DC operating point of a tube: the (Va, Ia) pair where the loadline crosses the bias-grid curve, before any signal is applied. Determines headroom, distortion symmetry, and tube…
Loadline: Graphical line on the plate-characteristics chart representing all (Va, Ia) the load allows. Slope = -1/RL. The intersection with the grid-voltage curve gives the operating point.…

Parameters (5)

μ (mu): Amplification factor: the ratio of plate-voltage change to grid-voltage change at constant plate current. A figure of merit for triodes (e.g. 12AX7 μ=100, 6SN7 μ=20). Mathematicall…
gm (transconductance): Plate-current change per volt of grid-voltage change, at constant plate voltage. Expressed in mA/V or μmhos. The slope of the Ia/Vg curve at the operating point. Higher gm = more d…
rp (plate resistance): Internal AC plate resistance: the slope of the Ia/Va curve at constant grid voltage. Lower rp = more authority into the load, higher damping factor. Triodes typically 1-80 kΩ; pent…
Pa (plate dissipation): Power dissipated as heat at the anode: Pa = Va × Ia. The maximum rated Pa is the absolute thermal limit; staying ≤70% of max preserves tube life. Exceed it and the plate glows red,…
Damping factor: Ratio of speaker impedance to amplifier output impedance: DF = Zload / Zout. Higher DF means tighter bass control. SET amps without feedback typically DF=2-5; well-designed PP-pent…

Phenomena (4)

Miller capacitance: Effective input capacitance of an inverting amplifier: Cin = Cgp × (1 + gain). In high-mu triodes (12AX7) the small grid-plate capacitance is multiplied by ~100, severely limiting…
NFB (negative feedback): A fraction of the output is subtracted from the input. Reduces distortion, lowers output impedance (raises damping factor), flattens frequency response — at the cost of gain and, i…
Microphony: Audible response of a tube to mechanical vibration. The internal structure (cathode, grid wires) vibrates and modulates the plate current. A well-designed tube has tightly-spaced g…
Crossover distortion: Distortion in push-pull class-B/AB stages at the zero-crossing of the signal, when one tube turns off before the other turns on. Audible as harsh upper-harmonic content at low leve…

Power (3)

OPT (output transformer): Steps the high impedance of the tube plate(s) down to the loudspeaker's 4/8/16 Ω. Quality determines bass extension, top-end air, and damping. Push-pull OPTs have a centre-tapped p…
Choke: Iron-cored inductor used in B+ filtering. Replaces (or supplements) a resistor between filter capacitors. Drops less DC voltage and gives much better hum rejection than an RC filte…
B+ (HT): The high-voltage DC supply that feeds the plates and screens. Typical values: 200-400 V for preamps, 350-500 V for output stages. Lethal — always discharge filter caps before worki…

Simulation (1)

Spice / Koren: Norman Koren's empirical triode model (1996) is the de-facto standard SPICE model for vacuum tubes. Captures soft knee and grid-current onset using ~6 parameters (μ, Ex, Kg1, Kp, K…

Topology (7)

Cathode follower: Triode stage with output taken at the cathode, plate grounded for AC. Voltage gain ≈1, very low output impedance (≈1/gm), high input impedance. Used as an impedance buffer between…
Common cathode: The classical voltage-amplifier topology: signal enters at the grid, output taken at the plate, cathode grounded (or AC-grounded via bypass cap). Inverting, gain = -gm × (rp ∥ RL).…
Cathodyne: Phase-splitter topology: equal resistors at plate and cathode produce two outputs of equal amplitude and opposite phase. Excellent balance, but limited drive for the next stage's g…
Long-tailed pair (LTP): Two cathodes joined to a common large 'tail' resistor (or current source). Produces balanced phase-split output with active gain in both halves. Used as the input stage of differen…
SRPP: Two stacked triodes where the upper acts as a dynamic load (constant-current-like) for the lower. Lower distortion than a resistor-loaded common-cathode stage, but heavily debated…
Mu-follower: Bottom triode (common-cathode) loaded by an upper triode wired as a cathode follower acting as a near-constant-current load. Very low distortion, gain near μ, lower output impedanc…
Ultralinear: Pentode/tetrode output stage where the screen grid is connected to a tap on the output transformer (typically 40-43% from the plate). Lower distortion than pentode mode, more power…

Troubleshooting (1)

Hum loop: 60/50 Hz hum induced when two pieces of equipment connect their chassis grounds via two paths (signal cable + mains earth). Forms a tiny transformer secondary that picks up mains s…