The Model 111 Gm and Em Tube Tester introduced to the
electronics industry the most complete tube evaluation
available, other than by expensive and complicated
laboratory Bridge Systems.
The Model III makes the following general tests:
Gm — Mutual Plate Transconductance.
Em — Plate Emission.
Leakage and Shorts.
Gm (Mutual Plate Transconductance) is the ability of the
grid of the tube to control its plate current.
Mathematically it is a small change in plate current divided
by the change in grid voltage required to make the plate
current change. The plate voltage is kept constant. From
this it can be seen that the Gm is a most important
measurement - especially in video, I.F. and power amplifiers.
The Gm test is made by applying pulsating DC voltages as
follows:- Positive to the plate and screen; negative to the
grid. An AC signal voltage is also fed to the grid which
causes an AC increment to appear in the plate current. It is
the measurement of this increment which ascertains the Gm of
the tube. These are actual average Gm measurements and are
Em (Plate current measurement) of a tube is, as the name
imnlies, measurement of the tube plate current. Although the
voltages applied to the plate are swept, the measurement is
still primarily static. It is an indication of the cathode
current (which, of course, controls self bias) and also the
plate resistance, etc. In this test separate voltages are
again fed to the plate, screen and arid of the tube. The
voltages are AC, the current is DC since the tube rectifies it.
SELECTING THE PROPER TEST
It may be said that the more tests made, the more correct
the analysis of the condition of a tube. |t is necessary,
however, to occasionally look for a faster means. With this
idea in mind, PRECISE has * (Asterisked) the most important
single test for each tube or section thereof. |f you do not
wish to make both the Gm and Em tests, we recommend that you
select the one with the * in back of its listing on the roll
TESTING A TUBE FOR Gm
1- Select the tube type under the heading marked 'TYPE'
abovo the Roll Chart. Gm tests are indicated by 'Gm' in the
2- Set the FILAMENT Switch to the same value as shown under
the heading • FI L ’ - The top value listed is the Filament
voltage. The value directly beneath it is the Filament
current. Measuring the current is discussed in a later section.