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Re: ignitron tubes
>>From bigr-at-teleport-dot-comTue Jun 25 22:01:57 1996
>Date: Tue, 25 Jun 1996 13:29:26 -0700
>From: bigr-at-teleport-dot-com
>To: tesla-at-pupman-dot-com
>Subject: Re: ignitron tubes
>Tesla List wrote:
>
>> >hi! wanted to see if anyone in the group has had any experience using
>> >ignitron tubes in "big coils",i have a couple that i'm going to use on
>> >my 18,000 watt coil, anyone got any ideas?
>> > tnx bill ...hb-at-earthlink-dot-net
>
>> Hi Bill,
>
>> I believe that the general concensus 'round these parts' is that
>> ignitron tubes are not suitable for use in Tesla coils of any size
>> because the turn off (deioinization) time is far too long. This is a
>
>Someone had asked several weeks ago about the identity of some mystery
>tubes. They were found to be thyratrons, but the normal mercury vapor
>type. However, as I said, I understand that there are "hydrogen"
>thyratrons that are very fast and would be suitable for Tesla coil gap
>replacement. (although I wouldn't want to use one that had 02 leaking
>into it, ha) But, I've never seen any on the surplus market- nor do I
>have any specs for any- were probably only used for exotic gov
>projects back during the Cold War. Rob.
Rob, Bill Henderson, All,
A number of high power pulse discharge systems were developed at the
Radiation Research Laboratory at MIT during WWII to repetitively
hammer magnetron tubes with very precisely timed power bursts of high
voltage DC current. Successful techniques included triggered spark
gaps (open and enclosed), rotary breaks, triode vacuum tubes (hard tube
pulsers) and hydrogen thyratrons. There is an excellent publication
which is hard to find but you should check your university library.
Pulse Generators, edited by G.N. Glasoe and J.V. Lebacqz, McGraw-Hill
1948. Chapter 8 devotes itself to rotary break performance.
The trouble as I see it for us Tesla coil folk is that our
application is too dissimilar to a magnetron tube. In a Tesla coil
we need a bi-directional switch that can go in series between the RF
capacitor (in our case also serves as the energy storage device) and
the RF tank coil.
In the magnetron application the RF tank circuit is all pre-wired up so
to speak, (the parallel resonant cap and coil, oscillator tube,
feedback circuit etc.) is the tube, (yes I know this is not how a
magnetron tube works but it is adequate for the point I'm trying to
make here). These entities are not separated. We see the magnetron as a black
box with a power supply connection in, and an RF output port. We merely
have to charge our energy storage reservoir, in this case a PFN (pulse forming
network) which is like a capacitor, with DC, then we close a switch grounding
the supply end of this PFN (the switch will only see DC without reversal)
and the other end of the PFN kicks the magnetron which is a black box
oscillator as mentioned before that only needs HVDC input in order to output
CW RF as long as it is powered by the finite shape controlled pulse
from the PFN. Did anyone catch that I said a pulse of continuous
wave RF?
Some of us build vacuum tube Tesla coils which are merely large,
self-excited CW oscillators with our high voltage secondary planted in the center
of the primary coil. Vacuum tube T.C.'s can operate with relatively tiny
tank circuit capacitance size because we are not storing, then
dumping power disruptively like in a spark gap driven system, the
output power escapes at the same rate as energy is delivered into the
tank circuit by the vacuum tube.
Now here's an idea. Get a robust
triode designed for high pulse power (the plate will be relatively
small in dissipation because of the low average power, compared to a very
heavy filament structure which is needed to source lot'sa amps during
the short on time). Use one of these as your RF oscillator tube. Have your
Vacuum tube T.C. all hooked up in the normal
way, but remove the high voltage AC or DC plate power supply and have some
friends help you push it out of the way. Now, fabricate a line
pulser type power supply where you will charge a low ESL pulse capacitor
(perhaps 0.1- 0.5 mfd) from a DC source through a supply protecting choke
or resistor, and charge that cap up to the voltage rating of the
pulse triode used (perhaps 7-10KVDC). Use a rotary break, or yes a hydrogen
thyratron, or yes even a mercury thyratron
to short the supply side of the storage cap to ground when
repetitively triggered. The idea is to send stored packages of DC
energy off to the tube oscillator circuit which is setup to deliver
punishing peak power when kicked. You will have to figure out how to
complete the DC circuit around the capacitor for the charging mode to
operate. You could use a large series choke from the output side of
the PFN to ground. This choke will be in parallel with the load
represented by the oscillator and must be capable of surviving the
repeated high voltage pulses thrown across it. Matching impedances
here and in the PFN to the load impedance of the oscillator and the
rep-rate of the pulser will be critical to get acceptable performance. The storage
capacitor must be sized small enough so that it can dump all
of its stored energy into the oscillator during the swich commutation
period. Building up a true line pulser circuit employing several
capacitors with series inductors will give some waveshape control of
the tail of the delivered pulse which is necessary in radar
apllications but I don't think is necessary for making sparks. If
you have access to real radar PFN's you could try those.
The results should be a much more impressive
discharge streamer length than is normal for a vacuum tube powered
T.C. I haven't tried this yet but have had it in the back of my mind
as a someday project for a while. Bill Henderson, an 18,000 watt
version of this system as just described ought to be pretty
impressive!
Comments are welcomed.
Happy Coiling, etc!, rwstephens