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Re: Pulsed Tube Coils
>.In a message dated 96-12-31 13:55:40 EST, you write:
> Hi John,
> I found your recent posts on tube coils quite interesting. There is one
>idea I've wanted to try but haven't as yet, maybe you have and could cue
>me in on the
>practicality of it.
> I was thinking of building a tube coil using a pair of
>572-B's (basically heavy duty 811's) with a filtered DC
>plate supply of about 2500 volts, the power supply would
>have a output filter capacitance of about 15 UFD and the
>circuit would be the typical link coupled feedback setup.
> But I was going to use a square or triangle wave signal
>to drive a power mosfet (IRF-511) switch in the filament
>center tap (cathode return) to gate the tubes conduction
> much the same as what a variable rate/dwell rotary gap
>would do in a disruptive coil, or even use a audio
>source to modulate a fixed bias reference in the cathode
>return. Any thoughts on this approach?
Mark Graalman
>>
Hi Mark,
Thanks for the suggestion, I did some preliminary tests of your idea today
at a low power, using a standard (12" spark) AC tube TC, and the results were
excellent! I used a horizontal output transistor because I didn't have a
power mosfet. I connected the collector to the tube filament, and the
emitter to ground (after of course opening the original filament-to-ground
connection). Then I connected my pulse generator across the transistor base
and ground.
I only turned up the main TC power part way because I was concerned about the
possibility of kickback destroying the transistor, the (TV-type) filament
transformer, or the pulse generator. But I did obtain a 5" spark, which
showed a nice staccato pulsing.
I did not yet try the technique on a DC Tesla coil.
In general, your technique may greatly simplify the pulse control circuitry
compared with what I've been using, and eliminate any potentially undesirable
interactions with the grid-leak network. It remains to be seen if a DC
pulsed tube coil using this system will still require a tetrode and AC (or
other up-ramping voltage) on the screen grid for long spark production. I
definitely recommend pursuing this cathode-type control technique. If you
try this technique Mark, I'd definitely be interested in your results.
John Freau