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Re: Really big VTTCs

Original poster: "Steve Ward" <steve.ward@xxxxxxxxx>

Hi David,

I have played around with larger SSTCs (running quasi CW, much like a
VTTC).  With my larger unit (12"x18" secondary) i had to pump about
7kW into it to reach 36" sparks at 60 pulses per second!  And, its
pretty safe to say that *most* of that 7kW was going into the sparks,
since my reasonably sized heatsink didnt get all that hot.  In CW mode
the coil has only been tested to 24" flames.  It seems that growing
long sparks in this quasi-CW mode is quite difficult when you get past
about 2 feet of spark.  The RF envelope (or, more importantly, the
manner in which energy is delivered to the spark) might play a
critical role in making the sparks grow longer instead of just
thicker.  In a test with a DRSSTC running 10mS pulse widths (so again,
semi CW as the system reached steady state for many mS) i was able to
develop 4-5 foot gnarled streamers.  The sparks ate up about 150J per
shot!  I was limited to a few shots per second due to this.

Its my opinion that however you wish to switch power into the primary
circuit is not what matters (tubes, MOSFETs, IGBTs...) provided the RF
energy envelope is the same.  Lower voltage drivers will simply
require a lower impedance tank circuit (so you have more current, but
less primary inductance, so its a wash), and this is why i think tube
drive and solid state drive are able to produce the same results
(provided you design them properly).

I think if a large VTTC is to produce "long" sparks, the drive will
need to be quite agressive, but of a lower duty cycle than most VTTCs
operate at (most just feed raw AC to the tube).  Perhaps shortening
the pulses from 8.33mS half-sines, to something like a 1mS square wave
will allow longer sparks to develop (given the same energy used up in
either case).  This will eventually make the sparks "thinner", and may
cause the sparks to lose some asthetic value.

Also, i wanted to mention that as coils get bigger, they lose the
sword-sparks.  I did tests (with a solid state drive) on a few
different coils.  The highest frequency coil gave the straightest
sparks (although, they werent as straight as the VT drive) while lower
frequency coils always gave the gnarled sparks.  Using lower F coils
on the VT drive also gave gnarled sparks (and they are typically
shorter).  Something i never tried was a really large secondary with
few turns of wire (so the frequency was still high).  If this works,
then maybe it is possible to keep the sword streamers at very high
power levels.  In this case, VT drive would probably beat out solid
state, as tubes are considerably faster for this application.

Steve Ward

On 1/21/07, Tesla list <tesla@xxxxxxxxxx> wrote:
Original poster: David Speck <dave@xxxxxxxxxxxxxxxx>


I asked this question a couple of years ago, but never really got a
definitive answer.
I've seen experimenters construct disruptive coils with secondaries
from a few inches long to 60 feet tall.  Current SSTCs are
approaching or surpassing the performance specs of the disruptive
coils every day.

By comparison, VTTCs seem to have topped out at a 4" x 20" secondary,
driven by one to four 833A tubes.  The consensus seems to be that you
can get just about as good performance from one 833A as with 4 of
them, if you design the coil right.  Perhaps 36" discharges seem to
be the limit for VTTCs on a good day.
Is there some physical or electronic limitation to the design and
construction of a really big VTTC?  With the availability of big
surplus power tubes (10 kW or more) on eBay, and the possibility of
multiple parallel free MOTs for input power, is there a physical
reason why no one has built a 12 x 60 or 18 x 90 VTTC?
Ten foot sword like discharges would be neat to watch!