Tube testers, in general, provide a means of applying typical
operating voltages to the tube that is being tested. By providing a
potentiometers, a typical filament voltage, grid voltage, or plate
voltage is applied to the tube. The tube can then be given various
tests in this "typical" environment. Obviously, if the tube is to be
used in an application where it is not being operated with these
"typical" operating voltages, the tube tester will not be likely to
give an accurate indication of the tube's condition.
If a tube is gassy (that is, if the tube's vacuum is low or
absent because of insufficient evacuation or the release of gasses
imbedded in the electrode metals), grid current will flow in spite of
negative grid bias. Thus, a milliammeter or microammeter in the grid
circuit can be used to determine whether a tube is gassy.
current is detected when a tube's grid is negatively biased, then
that tube is gassy.
Figure 3-2 illustrates a typical circuit used in making a
Recommended operating voltages (Eg, Ef, Ebb
etc.) are applied to the tube and a 1-volt rms signal is applied to
The ac component of the resulting plate current is
indicated by the ac milliammeter.
circuit isolates the milliammeter so that it is not deflected by the
dc component of plate current. The transconductance is then equal to
reading (an rms reading) multiplied by 1000
For example, if the plate milliammeter reads 3-mA rms, the tube's
transconductance is 3 x 1000 = 3000 micromhos.