Valve / Tube Bias Calculator
Calculate a target plate current for common fixed-bias output valves, compare 50–70% dissipation points, and check measured tube current for imbalance. Built by Electronic Repairs UK for practical amplifier servicing.
Bias settings
Optional: check your measured currents
Enter up to four measured plate-current readings in mA.
What the numbers mean
Why is 70% shown?
For many class-AB fixed-bias guitar amplifiers, 70% of the valve’s maximum plate dissipation is commonly treated as an upper reference point at idle, not a mandatory target. Some amplifiers are intentionally set cooler.
Why can a bias probe read slightly higher?
Many cathode-current bias probes measure cathode current, which can include screen-grid current as well as plate current. This calculator reports the plate-current figure implied by the dissipation formula.
Does every EL34 or 6L6GC use exactly the same setting?
No. The correct operating point depends on the amplifier design, actual plate voltage, valve type, screen conditions and the manufacturer’s intended operating range. Always consider the amplifier circuit, not just the valve name.
Want these tools in your pocket?
Valve Bias Pro adds valve and amplifier libraries, multi-valve comparison and saved workshop jobs. Lifetime Pro is £3.99 one-time.
Prefer us to check the amplifier?
Electronic Repairs UK offers valve-amplifier diagnosis, bias checks and professional valve testing in Telford.