VACUTRACE Vacuum Tube Curve Tracer
12
Technology
Generating Curves
VacuTrace sweeps the characteristic curves of a vacuum tube by applying plate,
screen and grid bias voltages and measuring the resulting cathode current. A low
value resistor shunts the cathode to ground converting the current into a voltage
that is then amplified and sent to the Y channel of the oscilloscope. The plate
voltage is ramped up and down and (an attenuated copy) is sent to the X channel,
thereby “drawing” a curve on the oscilloscope’s display. The update rate determines
image flicker and if fast enough, the curves will appear continuous.
A set of curves is formed because the grid voltage changes to a new value every
time the plate reaches 0V. The grid is stepped to eight different levels starting at
0V. The oscilloscope photo below shows the relationship between plate and grid
voltages.
The peak plate voltage is determined by any of the three limit controls. When one of
these limits is reached the ramp is reversed back towards 0V. Normally the voltage
limit control sets the peak voltage. But often you may want to limit either peak
current or peak plate power, both of which can occur prior to the voltage limit. This
capability is to prevent tube damage.
Taking Measurements
Both static and dynamic measurements are done in Hold mode. Switching to Hold
mode turns off the sweep and sets the plate voltage to the present limit setting
(regardless of current and power limits).
Static voltages and current are measured using a standard analog-to-digital
converter (DMM) IC.