STSG, initial testing

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

Original poster: "by way of Terry Fritz <twftesla-at-qwest-dot-net>" <FutureT-at-aol-dot-com>


Hello all, 

I finally tested a STSG (sync-triggered spark gap) on my 
TT-42 TC.  So far the results are not quite right, but the 
system shows good potential. 

The controller is a K&B Electronics motor speed controller 
(similar to a lamp dimmer), which I modified by adding an extra 
triac to make it somewhat more stable.  I also did not use the 
DC output option of the device.  I used 5 ohms resistance 
on the output, and fed this through a 1uF cap, because these 
values seemed to work best.  I limited the range, because if 
I turn the pot too far, the firing becomes unsteady.  I looked 
at the output with the scope, and it's rather steady phase-jitter-wise. 
The coil is a GM HEIC. 

The actual spark gap is made from heavy copper pieces with 
thick silver faces about 1.2" square.  The trigger electrode is 
made from 1/8" tungsten rod.  I seem to have to set the main gap 
spacing at about 5/8" for best results.  I tried connecting the coil 
core to ground, and connecting the HV output terminal to the trigger 
electrode, but this forces the HEIC output to seek ground 
through the safety gaps and they fire mostly continuously 
without the coil running, so I couldn't use that method. 
Instead, I connected a 500pF, 30kV cap between the HEIC 
core and each side of the main gap.  Two caps in other words. 
This method worked a lot better.  At first I didn't use any air, 
but the coil works better when I let a small fan blow on the gap. 

I can't turn the power up all the way because the firing becomes 
unsteady.  I think I don't have enough phase range, because 
I'm right at the end of the range of the pot, and I know the phase 
needs to be changed as the power is raised.  Nevertheless, I can 
turn the power up to about 120 volts (instead of a usual 140 volts). 
I get a strange surging or unsteady firing up around 120 volts. 
I didn't view the scope with the coil running, so I'm not sure yet 
what's happening.  It may simply be that the phase range is 
not enough at that point.  The TT-42 seems to demand a phase 
position that's a little different than most coils anyway. 

In any case, the coil sounds mostly good, and the sparks look good, 
and are almost the normal length so far, but I didn't measure them. 
The actual spark gap arc is ferociously bright.  I can't tell how loud 
it is, because the coil is so loud anyway.   

During the initial tests, I got a bunch of racing sparks on the 
secondary which burned some spots on the wire, but the coil 
still works.  I don't have any photos of any of this. 

There is no doubt that a STSG system is easier to build than 
a sync rotary, from a physical construction point of view anyway, 
and it promises to offer good results in my coil.  Tests will continue.   
Theoretically the gap losses will be higher with the TSG, but 
we'll see if it makes much difference in the actual performance. 

I commend Marc M. on his design and advocacy of the STSG. 
Thanks too to Terry and all the others who have advanced the 
STSG art.  The STSG has taken its rightful position alongside 
the other gap options. 

For those who are not familiar with my TT-42 coil, it can be 
seen at:    

  http://hometown.aol-dot-com/futuret/page3.html 

I think you have to click on spark gap coils. 

(I normally run it with a 120 bps sync rotary) 

Cheers, 
John