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Re: Pulsed Tube Coils
In a message dated 97-01-06 00:55:41 EST, you write:
<<
>>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
> >>
>>>snip
>> 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)
>
> >snip
>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.
>>>>snip
> >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?
>>
Mark,
I knew I forgot something in my my posts---I used a squarewave for all
tests--it's all my generator can provide. And Yes, I suppose that if the
proper waveform is fed to the "cathode lifting" control transistor, this may
make it unnecessary to use the AC on the screen grid. Then DC on screen grid
would be OK, or use a triode. But if duty cycle is high, a lot of power will
be drawn.
BTW, I did try connecting my negative control pulse to the screen, instead of
the control grid in one of my earlier experiments--and the technique worked
well, and in fact eliminated the potential problem of interaction between the
neg. control voltage and the grid-leak network.
I also like your idea of using cathode bias to eliminate the need for a
negative control voltage or screen voltage to cut off the tube. I'm not sure
what zener voltage rating would be needed here, may need to be more than a
few volts--if this is what you generally had in mind--and I suppose you'll
lose power due to cathode degeneration. There are certainly plenty of
possiblilites here.
In general, when pulsed, a DC coil and an AC coil will sound about the same,
this is because if only one half cycle is passed, the ear will hear this as a
"pop" rather than a tone. depending on the pulse rate, the sound can be like
a chain saw revving up, or like a rat-tat-tat, or staccato sound. If you
use voice or music modulation, that's another story, and I can see why you
would want the DC for a quiet coil starting point. I think the DC with audio
modulation should work very well, but sparks will probably be short, and the
coil will probably draw a lot of power, you should hear the audio coming out
of the spark. I never tried this but it should be pretty cool. Perhaps
someone else has tried this? The coil could be voice actuated, so the tube
can cool between words, music will probably draw a lot of power, you may have
to keep the power low, or use a giant tube to keep it cool, etc.
It sounds like your onto something here with these ideas, I'll be interested
to hear what you come up with once you get your lab back on line. I don't
know just how much I'll be able to do along these lines, always glad to hear
of any new ideas that you'd like to share.
Happy (modulated) coiling,
John Freau