Bench tool · draft

Tube Tester / gm Calculator

Enter your bias probe's raw readings at two pot settings — gm is calculated for you.

Live calc
Where these numbers come from Ip at each point is read directly off your DIY Bias Probe's ammeter (the probe breaks the plate pin and puts a DMM in series in mA mode — true, loaded plate current, no math needed for that reading). The resistance value is your bias circuit's setting at that same moment (e.g. this site's adjustable-bias mod's floor-resistor-plus-pot total). Pick your tube type below to auto-fill its datasheet gm figure from the site's tube database, if it's listed — everything else, including gm itself, is calculated automatically. DIY Bias Probes Champ-Style Amp Tube Tester & Bias Mod

Point 1 — normal bias setting

Derived grid voltage: Vg1 = −(Ip1 × R1) = —

Point 2 — shifted bias setting

Derived grid voltage: Vg2 = −(Ip2 × R2) = —

— gm (mA/V)
— gm (μmhos)
— ΔIp
— ΔVg
Enter a datasheet gm figure above for an automatic health comparison.

gm = (Ip2 − Ip1) / (Vg2 − Vg1), with Vg = −(Ip × R) at each point — the same self-bias relationship a cathode-biased stage's bias resistor already relies on, computed directly from the bias probe's current readings.