Hi,

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.

I'll bet my 1928 Hickok really "slams" that grid signal. It's so old that it
has a 5-pin adapter with grid cap for them new-fangled screen-grid tubes,
tetrodes or whatever they called 'em.

It seems to use a wattmeter type of meter movement (dual coils) to multiply and
correlate the grid drive and plate output to compute the gm. Very cute unit.
--Mike K.

Oddly enough, that model (AC-47) uses a 2.5VAC grid signal. And
yes, it uses a dynamometer meter movement that measures AC
milliamperes directly (6.25mA F.S.). It also has a DC plate
milliammeter.

73, Alan