Target bias current per tube
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What biasing does
In a push-pull class AB tube amplifier, the power tubes conduct a small idle current even with no signal. This bias current sets where the tubes operate. Too cold (low current) and the amp sounds thin and harsh with crossover distortion; too hot and the tubes overheat, wear out quickly and can fail – sometimes taking other components with them. New power tubes almost always need the bias checked and adjusted.
Bias current (mA) = maximum plate dissipation (W) × target % ÷ plate voltage (V) × 1000
Typical values
| Tube | Max plate dissipation | Example at 450 V, 70 % |
|---|
| EL84 | 12 W | ≈ 19 mA (at 300 V: 28 mA) |
| 6V6GT | 14 W | ≈ 22 mA (at 400 V: 25 mA) |
| EL34 | 25 W | ≈ 39 mA |
| 6L6GC | 30 W | ≈ 47 mA |
| KT88 / 6550 | 42 W | ≈ 65 mA |
Measuring bias
- Cathode resistor method: many amps have a 1 Ω resistor from each cathode to ground – 1 mV across it equals 1 mA of cathode current (plate + screen).
- Bias probe: an adapter between the tube and socket with a built-in 1 Ω resistor.
- Output transformer shunt: measures plate current directly – accurate but dangerous, for experienced technicians only.
Because cathode readings include screen-grid current (often 5 – 10 % of the total), technicians aim a little higher on cathode readings than on pure plate-current readings.
Fixed bias vs cathode bias
Fixed-bias amplifiers (most 50 – 100 W amps) have a bias pot or a fixed bias network that sets a negative grid voltage – these need adjustment when tubes are changed. Cathode-biased amps (many small EL84 and 6V6 amps) are self-adjusting; you only need to check that dissipation isn't excessive.
Safety first
Tube amplifiers store voltages that can kill, even long after they've been switched off. Never work inside an amplifier unless you've been trained to discharge filter capacitors and work safely with high voltage. A qualified amp technician can bias and check your amp quickly and affordably.
Frequently asked questions
What should my EL34 bias be?
At about 450 V and 70 % of 25 W, around 39 mA per tube (or roughly 41 mV on a 1 Ω cathode resistor including screen current).
What is 70 % bias?
Setting the idle plate dissipation to 70 % of the tube's maximum rating – a common target for class AB push-pull amps.
Do I need to bias new tubes?
In fixed-bias amplifiers, yes – check and adjust bias whenever you change power tubes.
What happens if a tube amp is biased too hot?
Tubes run at excessive dissipation, plates may glow red, life is shortened and components can fail.
Is biasing a tube amp dangerous?
Yes. Amplifiers contain lethal voltages; biasing should be done by qualified technicians.
About BiasPoint
BiasPoint is an independent, free reference calculator for tube amplifier biasing, based on published maximum plate dissipation figures. Tube amplifiers contain lethal voltages even when unplugged – biasing should only be done by qualified technicians. We are not affiliated with any amplifier or tube manufacturer.
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Last updated: October 28, 2026
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