mA
Valve
Operating class
Measurements
0 V for fixed bias
1 Ω sense resistor: 1 mV = 1 mA
Idle Dissipation Meter
Plate dissipation
15.0 W
In the sweet spot
Plate current
33.3 mA
Va-k 450 V
1 Ω sense reading
35.0 mV
Target (50–70%)
29–41 mA
cathode current
100% Pa70%50%
Engineering Notes
How it works
"Biasing" a valve amplifier means setting the idle current of the output valves: the current that flows with no signal. The power this turns into heat at the plate is simply:
Pa = (Va − Vk) × Ia
Most technicians measure cathode current, which includes the screen grid's share (typically 3–7% at idle for pentodes and beam tetrodes), so it is subtracted to obtain the true plate current. Measuring across a 1 Ω cathode resistor is convenient: every millivolt equals one milliamp.
How hot?
- Class AB push-pull: 50–70% of maximum dissipation is the classic window. Colder bias gives crossover distortion ("sputtery", thin); hotter gives a sweeter tone at the cost of valve life.
- Class A / single-ended: the valve idles at the full current it will ever need, so 70–90% (even 100% for some designs) is normal.
Always confirm the plate voltage at idle with the bias current set; B+ sags as current rises. Lethal voltages are present: use one hand and insulated probes.