IEC 60151: Electronic Tube Measurements — Precision Determination of Vacuum Tube Health

Measuring Electronic Tube Performance — The “Health Check” of a Vacuum Tube

IEC 60151-28 specifies electrical property measurement methods for electronic tubes. A tube transconductance (gm) — the ratio of anode current change to grid voltage change — is the core parameter measuring amplification capability.

Measurement method: With anode and screen voltages fixed, superimpose a small-signal AC voltage (typically 100 mV RMS, 1 kHz) on the grid bias, and measure the AC component of anode current. gm = ΔI_a/ΔV_g. Typical small-signal tubes (12AX7): gm≈1.6 mA/V; power tubes (EL34): gm≈11 mA/V. gm gradually declines with ageing — when it drops to 60–70% of the initial value, the tube is generally considered end-of-life.