Valve amplifier bias is the operating point of the output valves when the amplifier is idling. In a fixed-bias guitar amp, the key figures are plate-to-cathode voltage, plate current and the valve’s maximum plate dissipation.
For a quick calculation, use our free Valve / Tube Bias Calculator.
The basic plate-dissipation calculation
Plate dissipation in watts is calculated from plate-to-cathode voltage multiplied by plate current in amps. For example, a valve operating at 450 V and 35 mA is dissipating 15.75 W: 450 × 0.035 = 15.75 W.
If that valve is rated for 25 W maximum plate dissipation, 15.75 W is 63% of the rating. Our Plate Dissipation Calculator can calculate this directly.
Why people talk about 70%
For many class-AB fixed-bias guitar amplifiers, 70% of maximum plate dissipation is commonly used as an upper reference point at idle. It is not a universal target. Some amplifiers are intended to run cooler, and the circuit design, valve type, screen conditions and manufacturer’s service information all matter.
Plate current and cathode current are not always the same
Many plug-in bias probes measure cathode current. Cathode current includes plate current and can also include screen-grid current, so the displayed figure may be slightly higher than true plate current. That distinction matters when comparing a probe reading with a plate-dissipation calculation.
Cathode-biased amplifiers
Cathode-biased output stages are handled differently. Current can be estimated from the voltage drop across the cathode resistor, using Ohm’s law. If several valves share one cathode resistor, the total cathode current is divided between them. Screen current should then be considered when estimating plate current. Use our Cathode Bias Calculator for this method.
Valve ratings vary
Do not assume every valve carrying the same family name has exactly the same maximum rating. Ratings can vary by manufacturer and by historical version. Always verify the datasheet for the exact valve being fitted.
High-voltage safety
Valve amplifiers can contain lethal voltages even after they have been switched off and unplugged. Bias measurement and adjustment should only be carried out by someone competent to work safely around high voltage.
If you would rather have the amplifier checked professionally, contact Electronic Repairs UK for valve-amplifier diagnosis, bias checking and valve testing in Telford.