Hi,

Ah, I see. But at least I can still get relative readings from tube
to tube, I suppose. (i.e. Get a scale reading for a known good tube,
and compare others to it.) That is of some use to me--if the readings
are somewhat linear.

Yes I think the readings are generally proprtional to Gm, which is
why the good-bad scale works. It's possible however that variations
in the operating point could swamp the Gm variations: in other words,
a tube that happened to draw more plate current for a given grid bias
might test unusually strong even though its Gm was not higher.

Any tester made after, say, 1950 will probably use a lower grid
signal than 5V. The lower the better, for low-bias tubes like the
12AX7.

73, Alan