So I have finally got my small valve coil working with a square wave
generator driving a transistor in the cathode/ground circuit.
I can vary the pulse rate and duty cycle, and am pretty happy with the
outcome...
When I connect the scope accross the cathode and ground at the
transistor terminals, I get a nice clean square wave.
However, when I start to apply the plate voltage to the valve with a
variac, I start to see multiple parrallel traces directly above the
"on" or high portion of the wave form.
The higher the voltage on the plate, the higher they go, and the
output of the generator seems to get unstable. (you can hear random
popping sounds in the arcs, instead of a nice clean tone)
When I first turned it on, it was really bad, so I put the circuit
inside a grounded metal box.
It has improved things greatly, but still seems to be a problem at
high power levels.
Can anyone shed some light on how to solve this?
purhaps a lower impedence ground connection?
Currently, I have left the bypass cap connected between cathode and
ground (so it is effectively accross the transistor), but I'm not sure
this is correct?
One possibility:
One aspect of shielding enclosure/Faraday Cage application
is that stray signals can be induced/coupled to the
wires in and out. These strays 'ride thru' the shield
box on the wires and affect circuit operation. Some level
of filtering may be useful, from simple bypass caps to
more elaborate filters, in each line, as close to the
shield wall as possible. Ideally the filter is mounted
in/at the shield wall.
best
dwp
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