Free workshop tool
Cathode Bias Calculator
Estimate cathode current, plate current, plate dissipation and dissipation percentage from cathode voltage and resistor measurements.
Screen-current estimate matters. Current through the cathode resistor includes plate current plus screen-grid current. If you know the actual screen current, enter it. If not, the default is only an estimate and the calculated plate dissipation should be treated accordingly.
Android app
Valve Bias Pro
Use fixed-bias and cathode-bias tools alongside valve references, multi-valve comparison and saved jobs. Lifetime Pro £3.99 one-time.
View Valve Bias Pro →Formula
How the cathode-bias calculation works
Step 1 — total cathode current
Total cathode current = cathode voltage ÷ cathode resistor. The calculator converts that result to milliamps.
Step 2 — current per valve
When multiple output valves share one cathode resistor, the total current is divided by the number of valves, then the entered screen-current estimate is subtracted to estimate plate current.
Step 3 — plate dissipation
Estimated plate current is multiplied by plate-to-cathode voltage to calculate plate dissipation in watts and as a percentage of the entered maximum rating.