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Better small-cap success.
Okay, now I got my "real" capacitors: Rifa PME 264, 660 VAC/1600 VDC
with impregnated-paper dielectric. I was a bit worried about dielectric loss,
but it turned out fine. Now I have two strings in parallel, each string has
15 of 100 nF caps in series, with 10 Mohm resistors in parallel with each
cap. So, now I can run for minutes (at least - see later in message) at a
time, and the caps aren't even warm to touch. I admit it - a 140 VA NST isn't
that much of a load on them, but that's for future testing.
The next thing I need to do, apparently, is make a better spark gap. What I
have now (two steel wires parallel) gets way too hot, I think it is causing
the phenomenon that when I start, sparks are about 10-15 cm, but then they
decay to 5 cm in about 20 seconds, and 2-3 cm in about a minute, at which time
the ground side of the gap is glowing _orange_ hot. So while the gap stays the
same, the spark will ignite easier, so the primary circuit voltage drops
all the time. Another problem with it is that when the spark gap is wide
enough, it fires for maybe a second when I switch power on. But then it stops.
I think this may be due to two things: the NST runs out of "juice", or
the current limiting kicks in and lowers the voltage. But with these caps,
during this short moment of higher voltage, I can get 20 cm sparks. Not a
lot to be excited about.
So now I start looking for better power supplies and copper tube :-)