In article , Alan Douglas
adouglasatgis.net writes:

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 nad plate output to compute the gm. Very cute unit.
--Mike K.

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