[TCML] Pulsed valve coil

["Carlos Van Camp" <carlos@xxxxxxxxxxxxxxxxxxxxxx>]

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 out come...

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 poping 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?

Thanks,

Carlos
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