Theory

Class A, AB, B, and Beyond

How bias point determines efficiency, distortion, and thermal behavior

The class of an amplifier refers to the fraction of the signal cycle during which the active device conducts. This single design choice cascades through every performance metric: efficiency, distortion, output impedance, and thermal management. In tube amplifiers, the class is set by the grid bias voltage relative to cutoff.

Class A — 360° conduction
Tube conducts entire cycle
Max efficiency: 25% (transformer) / 50% (ideal)
Lowest distortion (primarily 2nd harmonic)
Constant current draw — easy on power supply
Example: Single-ended 300B, 2A3
Class AB — 180°-360°
Both tubes conduct at idle, one cuts off at high signal
Efficiency: 35-50%
Odd harmonics appear at crossover
Current varies with signal — supply must handle peaks
Example: Push-pull EL34, KT88
Class B — 180° conduction
Each tube conducts exactly half cycle
Max efficiency: 78.5% (theoretical)
Crossover distortion at zero-crossing
Rarely used in audio — common in RF
Example: 807 push-pull transmitter
Class C — < 180°
Tube conducts less than half cycle
Efficiency: up to 85%
Extreme distortion — RF only with tuned load
Tank circuit reconstructs sine wave
Example: 4CX250B transmitter final
Calculator

Bias Point & Class Explorer

See how bias voltage determines operating class and efficiency

B+400V
I_idle60mA
I_max120mA
V_cutoff-45V
V_bias-30V
ClassAB
Conduction264°
P_diss/tube24.0W
Efficiency40%
P_out max32.0W
P_supply80.0W
Visual

Conduction Angle by Class

Reference

Amplifier Classes Compared

From single-ended Class A to RF Class C

ClassEfficiencyPowerTubesNotes
A SE20-25%3-8W typ300B, 2A3, 45, EL34SET audiophile favorite
A PP25-35%10-30WEL34, 6L6, KT88Studio monitors, hi-fi
AB135-50%20-100WEL34, KT88, 6550Most guitar/hi-fi amps
AB240-55%50-200WKT88, 6550, 813High power PA, bass amps
B50-65%100-500W807, 813, 4-65APA systems, transmitters
C70-85%10W-50kW4CX250B, 3-500ZRF transmitter finals only
Practice

Choosing the Right Class

Class A: When Quality is Everything

Class A single-ended (SET) amplifiers produce primarily 2nd harmonic distortion, which the ear perceives as warmth and richness. The tradeoff is severe: a 300B SET producing 8W dissipates 40W of heat in the tube alone. The output transformer must handle the full DC bias current without saturating — air-gapped cores are mandatory. Despite the low power, SET amps drive high-efficiency speakers (Klipsch, Altec, Lowther) to satisfying levels.

Class AB: The Universal Compromise

Most tube amplifiers — from the Fender Twin to the Quad II — operate in class AB. At low levels, both tubes conduct (class A operation, low distortion). As the signal increases, the tubes alternately cut off, transitioning to class B. The bias point is critical: too hot (class A) wastes power and shortens tube life; too cold (approaching B) introduces crossover artifacts. Fixed bias with a trim pot is standard for power tubes; cathode bias (automatic) is simpler but less efficient.

AB1 vs AB2: Grid Current

In AB1, the grid never goes positive — no grid current flows, and the driver stage sees a purely capacitive load. In AB2, the grid is driven positive on peaks, drawing current from the driver. This extracts more power but demands a low-impedance driver stage (cathode follower or interstage transformer). AB2 is essential for high-power designs like the SVT (300W from 6×6550).

Quiz de synthèse

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Question 1 / 5

What is the maximum theoretical efficiency of a Class A single-ended amplifier with transformer coupling?