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So Maybe MOTs Don't Rule

Fellow Coilers,

My earlier enthusiasm for the twin MOT supply with two half-wave voltage
doublers has faded somewhat under the harsh light of experience.  After
packaging the MOTs in a metal chasis (US Govt. .30 cal. ammo can, perfect
fit!), I went through several pairs of blown doubler diodes.  The culprit
turned out to be the safety gaps across each MOT HV terminal.  Each time
the safety gaps fired, the doubler caps discharged thru the diodes and blew
them with overcurrent.  I tried discarding the safety gaps, but now the MOT
terminals simply arced over the the metal chasis due to inadequate
separation.  This had not been an issue before, because the safety gaps
were set more narrow than the chasis/MOT separation, so all transients
simply jumped the safety gaps.  Now the MOTs made their own safety gaps.
More blown diodes.

I added additional insulation, which cured the arc over problem, but now my
filter resistors began to fail.  I used the flat, wire-wound type which is
wound on an aluminum strip with a hole punched thru each end--great for
easy breadboarding but lousy for HV RF.  Transients simply arced to the
strip, bypassed the resistor body, and arced back onto the other end of the
resistor.  This HV RF defeated even my improved MOT insulation.  More blown
diodes (ARRRRGGGHHHH!).  So I modified the resistors by removing the metal
strip and suspended them from stiff wire leads.  I also moved the safety
gaps to the final output terminal of the supply, giving the transients some
place to go besides into my MOTs.  Ahhh, success.  No more blown diodes.
This foolishness used up almost all my junk box rectifiers.  Oh well...

Next problem:  way too much current and not enough voltage.  The MOTs got
scorching hot even with runs of less than a minute, and the filter
resistors were fairly smoking!  The culprit was my .91uF doubler caps,
which presented essentially a short circuit to the MOTs.  I cured this by
adding two more .91uF caps (from my junk box) in series with the first two,
effectively halving the series capacitance.  The doubler caps can now
charge to the peak output available from the MOTs, giving my voltage a
small boost (I can leave the static gap set the same as for my 9KV NST
supply).  This also reduced the current, so my MOTs run warm, but not hot,
and my power resistors run hot, but not glowing.  In other words, the
doubler caps do double-duty as capacitive current limiters.  Of course now
my MOT supply can develop "only" 1700VA, but it sure is much better
behaved.  I plan to idle my valuable NSTs and just run with the MOT supply
for my unstructured experimenting.

And the little 4" x 24" coil?  It crackles with an intensity that must be
seen & heard to be appreciated--it likes 1700VA from the MOTs much better
than 1080VA from the NSTs.  The 60Hz pulsing not only sounds cool, it also
affects the appearance and behavior of the streamers.  They are hotter,
jerkier, and more frantic than before.  Maybe MOTs don't rule, but they can
still make a very serviceable power supply if you have the patience to
tinker with them.

Next step?  I bigger MMC to better utilize my stiff MOT supply.

Best Regards from Middle Georgia,

Greg Hunter