Re: cx2708 thyratron

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "David Speck by way of Terry Fritz <twftesla-at-qwest-dot-net>" <dave-at-davidspeckmd-dot-org>

Bob,
The biggest problem is that a thyratron is a lot like an SCR, in that it 
conducts a whole lot better in the forward direction than in the reverse 
direction.  The specs for your unit indicate that it is good for 40,000 
amps forward, but only 1000 Amps in reverse.  I recall that there is a 
good intro to thyratron applications on the Marconi site, and it 
indicates that most thyratrons should not be subjected to large reverse 
currents, as they cause focal heating of the anode which leads to 
premature failure.  Certain tubes are designed to be more tolerant of 
this phenomenon that others, but none are really very good for the high 
current bidirectional RF AC that flows during oscillation of the tank 
circuit.

If you wanted to do a gap with Thyratrons, you would have to use two of 
them in anti-parallel, with opposite polarities - anodes and cathodes 
connected at each end.  You would need current steering diodes to make 
sure that each sees only forward current, and some floating switching 
arrangement that triggers both simultaneously while maintaining 
isolation from each other as well as the HV in the tank circuit.  I 
expect that there would be significant losses in the steering diodes to 
contend with.  Your tube requires a 1000 volt trigger, and, by itself, 
generating that would not be trivial.

Another problem is that thyratrons, like their solid state SCR 
equivalent, stay in conduction until their forward current drops below a 
certain value, and then, for some time after that.  I have a Thyratron 
drive board pulled from a LINAC system that drives a CX1159 H2 
thyratron.  It includes a hefty solid state SCR that drives a LC circuit 
to inject a reverse current pulse into the primary tube thyratron 
circuit to shut it off.  I have not had a chance to reverse engineer it 
sufficiently to determine how this shutoff circuit is triggered or 
driven, but I have seen similar circuits in block diagrams of thyratron 
circuits without any specific values or specs.

I believe that some of the big TC experimenters have tried such circuits 
in the past, but I have not seen any recent writeups on large designs. 
 Richard Hull demonstrated a small tabletop coil making hot and noisy 4" 
- 6" sparks using a modest size octal base hydrogen thyratron a few 
years ago at Ed Wingate's NY Teslathon.  I have a schematic for it 
"somewhere", but have not looked at in in several years.

I am slowly learning about thyratrons in my quest to recreate a table 
top version of  "The World's Largest Cosmic Ray Spark Chamber" that I 
saw at the '65-65 NY World's Fair.  I'm pretty sure I have all the parts 
I need -- just scored a 16" dia bell jar setup to build it in, and I 
have a couple of big scintillator blocks to to the triggering, plus the 
above mentioned thyratron, a very scary 35 KV Cap charging supply, and a 
couple of very big photoflash caps to make the discharge.  Now, I just 
need a year to make the electrodes and assemble it. :O)  If anyone else 
knows about this device, where it went, who built it, what the gas fill 
was, how it was triggered, what the electrode arrangement was, etc., I'd 
be delighted to hear from them.

HTH,
Dave
G2-1170

Tesla list wrote:

>Original poster: "bob by way of Terry Fritz <twftesla-at-qwest-dot-net>"
<yubba-at-clara-dot-net>
>
>hi all ,
>picked up a big thyratron today lying in the mud at the scrap yard. Does 
>anyone know if it would work as a  triggered gap? i  am not  familiar with 
>thyratrons but thought they had fast on slow off characteristics? data 
>sheet is 
>here 
>http://www.marconitech.co.uk/cgi-bin/download.cgi?thyratron&cx2708.pdf 
>should make a nice bang if nothing else.
>
>cheers
>  bob golding
>  
>