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TT-42 with larger toroid (tests and comments)
Original poster: "by way of Terry Fritz <twftesla-at-qwest-dot-net>" <FutureT-at-aol-dot-com>
I did some tests with a 6" x 24" smooth toroid installed onto the
TT-42 TC. I first removed the 4" x 13", but left the small 1.5" x 6"
toroid in place. The bottom of the new large toroid is 3/4" above
the top of the secondary winding.
I first tuned at 16.5 turns (76uH), this barely permitted the sparks
to break out at full power. Sometimes break out was delayed for
10 seconds or so. One time the breakout stopped, while the
coil was already breaking out. So this seems to be right at the
edge of breakout. The sparks seemed to be about 42" or so.
Next, I tuned at 15.5 turns. The sparks could not break out.
Then I tuned at 17.5 turns. The sparks could not break out,
so I added a breakout rod over the toroid edge. this permitted
breakout, and the sparks were longer at 44" perhaps or longer.
Next I replaced the breakout rod with a breakout bump made
of aluminum foil. The sparks broke out and I measured a 44"
spark. The spark hit the measuring wire with good energy, so
it could have gone further. Maybe they could have reached 46".
I didn't want to run the coil too long for fear of destroying the NST.
It has no protection networks.
I tried tapping at 18.5 turns, but the sparks were weaker, and
still needed the breakout bump.
It seems that a toroid which can only break out when in perfect
tune (if it is perfect), gives longer sparks when tuned outwards
even more, but with a breakout bump installed.
In all cases of breakout, there was only one streamer. This
proves that the spark length improvement with lower freq tuning,
was not due to a reduction in the number of streamers
No racing sparks occured during any of these tests.
Regarding the idea of tuning to compensate for streamer C,
someone on Pupman once suggested that the reduction in
Q with breakout should make the tuning rather non-critical.
If it's really very non-critical, then perhaps the lower tuning is
needed for a different reason, such as that mentioned by
Malcolm (offset to emphasis the lower split freq), or simply
to delay the breakout.
Whenever I saw racing sparks in the past, they were usually
just an inch or two long. The type that reach the full length
of the secondary occured only when the coil was severely
overpowered, or when the coupling was extremely tight.
If the racing sparks are caused by higher frequecies from
the primary, maybe they can be filtered out somehow?
An interesting test would be to use a sphere or near-sphere
which had the same C value as the 13" toroid, but had a large
ROC which gave only one streamer. This would tell us how
much spark length is gained by having only one streamer,
but keeping topload C the same.