Re: [TCML] Pulsed valve coil

[dave pierson <dave_p@xxxxxxxxxxx>]

> 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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