Re: Really big VTTCs

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: Sparktron01@xxxxxxxxxxx 

Hi Dave!  Happy New Year!

See my responses below...

 -------------- Original message ----------------------
From: "Tesla list" <tesla@xxxxxxxxxx>
> Original poster: David Speck <dave@xxxxxxxxxxxxxxxx>
>
> List,
>
> 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.
---------------------------------------------
You are comparing a relatively high Z vacuum tube (1 k ohm typical) to a
IGBT that is generally << 0.1 ohm even for a "sucky" device.  Switching
efficiencies in VT powered equipment will struggle to exceed 60-75%
(source to load), while IGBT's/FET's are often >95%.

One way to gain a power advantage with VT's is to use high voltage
power tubes (like hard switch radar modulators like 4PR**** and similar
where you can operate input voltages of >10kV.  But then you will deal
with x-ray generation, and capacitor voltage/current rating limitations.
Another design tack (which I have done on 3 separate occasions), is
to use push pull, where you will effectively double the tank voltage swing
and theoretically increase spark length by ~40% (sqrt of 2).
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> 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?
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See below insert from earlier TCML posting dated Jan 17, 2003
concerning known "large" (i.e high power) VTTC's that have been
built.

Robert Stephens from Ontario built a very large VTTC using
DC power supply and 3 air cooled 5kW plate dissipation class
tubes (BR1160) out of AM or MF short-wave transmitters, if up to full
capability would be able to deliver nearly 50kW (short duty)
to tank circuit.  Check archives for "Coronatron".  If Robert is
out there, maybe he can pipe in with an update (or at least the
present URL  documenting work).

Bill Wysock I believe posted a while back about tests performed
in Hawaii using a LF communication system for testing insulators
(<50kHz), notable in that power was very large VT transmitter in
the 100-200kW input range feeding a helix / resonator
through a matching line, length of arcs measured at 25kHz
breakdown alludes me but was on the order of 4-7'.

Several years ago, Ross Overstreet built a large VTTC using an
induction heating VT(3CX2500H3), tube is capable of up to
4kW short duty anode overload (2.5kW continuous), and
comfortably 10kW to tank, 20" of spark running arguably
1/10 of tubes capability.

Induction Heating VT's are ideal for VTTC service.
They are designed for severe overloads, ion back
bombardment, and overvoltage/overcurrent sins that
would detonate or melt down glass tubes.

David Sloan in 1934 used two hand built triodes running
push pull at 15kV AC to deliver 100-200kW into a resonator
for development 1MV for generation of intense X-rays for
early cancer treatment (predecessor to modern linac
radiation sources).  This work may be available on
hotstreamer or on the Web somewhere
(Review of Scientific Instruments).

I've heard from several sources that an extremely large
 bipolar VTTC was built capable of several 100kW input,
may be Internet / Folk / Technology Legend, never saw
any pictures, experimental composition, creators or
other technical details.

Largest VTTC I've personally witnessed was
John Freau's (HI!) triple and quadruple 833 coil, fed
by a 7.2kV PDT, capable of measured sparks in
the 38" range,  input power was in the 5-7kVA
range.  Ed Wingate has a similar VTTC using
graphite 833C's, capable of nearly 30" discharges
at ~ 4 - 5kVA input.

Realize, just having the tubes is not enough,
you have to in no particular order:
    1. Cool them.
    2. Power them
    3. Find _very_ high voltage - high power
         capacitors capable of continuous
        RMS tank currents in the 10's or
        hundreds of RF amps
    4. Start worrying about primaries catching
         on fire from thermal rise from RF currents
         and skin effects on primaries (high rms
         currents).
    5. Watch for fires caused by collateral
        induction heating effects around
        energized primaries and secondaries.
    6. Prevent primary to secondary breakdowns
        due to HV used in tank
    7. (NEW) If running above 10-12kV pk anode
        voltage, be concerned about soft X-ray
        generation and potential (no pun) exposure.
     8. (NEW) Power, controls and support hardware
        for personnel protection and to protect the tube
        itself.

Building a really large VTTC would be a much
more challenging exercise IMO then building an
equivalent power SG driven TC.

Today's state of affairs are no different then when I
originally penned this posting.  Just be aware the
"infrastructure" to build a really big VTTC is probably
10X greater then the cost / time required to locate
the tubes.  Tubes with 10kW  plate dissipation may
have filament powers in the kW range.  Filament
transformers (unless you hack a variac or MOT) will
be as hard and expensive to procure as the tubes.
Having access to high power MF transmitter/induction
heater components would give you a "leg-up" on
construction.
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> Ten foot sword like discharges would be neat to watch!

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I agree, just from a safe distance so you don't get
RF burns from contacting random metal...;^D

Regards
Dave Sharpe, TCBOR/HEAS
Chesterfield, VA. USA