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Re: Pulsed Tube Coils


On Sat, 4 Jan 1997 22:25:33 -0700 Tesla List <tesla-at-poodle.pupman-dot-com>
writes:
>Subscriber: FutureT-at-aol-dot-com Sat Jan  4 21:50:03 1997
>Date: Sat, 4 Jan 1997 14:55:26 -0500
>From: FutureT-at-aol-dot-com
>To: tesla-at-pupman-dot-com
>Subject: Re: Pulsed Tube Coils
>
>In a message dated 97-01-02 02:57:37 EST, you write:
>
><<       Hi John,
>   
>>snip
>  >>  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.   
>  
> >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
>  
>Mark,  All,      (update)
>
>Today I did two experiments:  First I turned up the voltage all the 
>way, and
>got a nice 18" staccato spark most of the time, but since I was still 
>using
>the non-sync, pulsed AC system, sparks sometimes "broke into multiple 
>spikes"
>and lost spark length, due to incorrect pulse timing.   
>
> Next, I hooked up the DC power supply, and ran the system.  I only 
>turned up
>the power part way so far (I don't like seeing my components burn 
>up!), but
>the sparks seemed to be maybe a little better than using by grid 
>controlled
>DC staccato (neither case here using the "AC on the screen-grid" 
>voltage
>technique).  What I mean by "better" is the sparks seemed less 
>"plasma-like".
> But it's hard to judge at low power.
>But the spark was always shorter and broken up into multiple fingers, 
>so I
>suspect that the AC screen technique, (or something else that gives 
>the same
>result), will still be needed for long DC tube TC sparks, using this 
>"lift
>the cathode" control method.
>
>John Freau
>
>
    Hi John,


  How do you think it may work out if the screen is 
supplied by a 500 -600 volt DC source rather than AC? Or
tying the cathode to ground through a 2 or 3 volt zener
for a slight amount of bias and switching the screen volt
age on and off with the transistor switch? Of course I'm
still talking about using DC on the plate circuit, I guess my ordinal
thoughts on this were to have a "silent"
output discharge then work on different modulation
techniques. It will be some time before I can do much in
the way of any experiments, my shop is loaded up, torn up, with
construction stuff from working on the house etc. 
 But the ideas still flow.......................

			Mark Graalman (WB8JKR) TCBA#1399

BTW, what type of waveform are you using to control the
switching transistor?