VACUTRACE Vacuum Tube Curve Tracer
11
Measurements
Hold Mode
Tube measurements are taken by switching to Hold mode. By doing so, the plate
voltage goes to the Voltage limit setting and the grid voltage goes to the Grid Steps
setting plus the offset from the Rate/Offset control. This determines the bias point
to operate the tube. You will also notice the 3½ digit LED display is enabled.
Setting the Output control to Vs reads the present screen voltage, or if set to triode
mode, plate voltage. Changing to Vg reads grid voltage. Use the combination of
Grid Steps and Rate/Offset controls to obtain any grid voltage from –0.5V to –70V.
Once the desired operating point is dialed in, switch Output to Ik to read the
resulting cathode current in milliamps.
Ratios
VacuTrace provides dynamic ratio measurements of great value to circuit designers,
namely transconductance gain and output conductance.
Transconductance (gm) mode measures the ratio of output Ik divided by input Vg
given in mA/V. The modulation of signals and division is all accomplished with
analog circuitry. You can see the modulation on the oscilloscope, centered about the
chosen operating point.
Similarly, gp measures the output conductance (1/rp) of the tube. It is the ratio of
output Ik divided by input Vp given in mA/V. Again, the modulation, or portion of
the curve being measured is visible in the display. VacuTrace always provides a
clear picture of what is being measured.
Other standard tube parameters are calculated by:
p
m
p
p
g
g
g
r
=
=
µ
1
