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TT-42 saga; NSTs, tuning, steadiness, etc.
Hello coilers,
I installed a 15/30 NST into the TC which gave nice 43" sparks with
a breakout bump on the 13" toroid, and nice multiple streamers about
40" long with no breakout bump. It turns out that the tuning was not
just right before which was contributing to the unsteady operation with
the PT's. I noticed that with smooth toroids....if you tap outwards too
much, the operation will be erratic. This is because the coil is then
tuned for longest sparks if they're formed. But they can't easily form
because the coil is mistuned before they break out. But if a break
out bump is installed, then the coil can be tuned (tapped) outwards
more, and still run OK. IWO, using a smooth toroid, you can tap
outwards more if there's a bump on the toroid. Maybe something
like this was happening in Malcolm's coil when it had intermittent
breakout problems with the large topload. (Malcolm, did you try
tapping inward at that time?) I wound up tapping at 15 turns instead
of 15.5 which was the old tune point with the PT. I may have had
to tune at 14.5 turns without the bump, I'd have to check my notes.
In any case, the display was quite spectacular for a 15/30 NST, and
the coil ran very smoothly while it lasted but......
Then the NST suddenly weakened on one side. The setup is some-
what of an LTR setup since the cap is 0.0147uF, but still the safety
gaps had to be set at 7/16" each which is kind of wide. I think this
coil with it's short secondary is not as efficient as my old coil, which
is to be expected from a Z match point of view. I made this coil
shorter deliberately to make it more portable. In any case this NST
may have been defective because the charging waveform showed a
strange dip near the middle of the charging waveform.
Next I installed a different 15/30 NST, this one showed a good
charging waveform, but unfortunately I forgot to hook up the ground
one time (I have the NST case connected to rf ground), and so the
TC got grounded through the core to the primary of the NST I
suppose, and the NST got very warm in the middle and the sparks
weakened.
I was thinking about these NST's in general, and it makes sense
to me that lower voltage ones may be more robust because they are
often the same physical size as a higher voltage ones. Thus, the low
voltage ones are being subjected to less voltage stress per unit area.
I've never had these kinds of failures using 12/30 NST's even using
reso-charging. (of course one of the 15/30's may have been defective
anyway, and the other I blew out from carelessness).
After that, I installed an old unpotted robust 12/30 NST with no safety
gaps, and had no more problems, but the spark is barely 42" long,
and is not quite as spectacular as with the 15/30 without the toroid
bump.
Overall, I'm satisfied with the performance of the overall design, and
the new small sync gap. I'm seeing very little erosion of the steel and
copper electrodes.
I still have to build a small base to make it portable, then it's finished.
For those who have not seen it, the photos of TC and sync gap are at:
http://members.aol-dot-com/FutureT/TT-42index.html
Cheers,
John Freau