Nick,
I snipped a lot here.
Do you have a safety gap across the caps or better yet across the
rotary or main primary spark gap system? I also have blown up a
capacitor or two. After much head scratching and help from another
coiler, I added a spark gap across the primary caps set to about
5/8" using about 1.5" diameter brass spheres. I was very surprised
to discover this new safety gap was firing (loud, like a rifle shot)
at about 60 or 70% of the main variac setting. I was experiencing a
primary resonance problem between the current limiting inductance
(welder) and the primary caps. The primary tank circuit voltage was
going WAY too high and overvolting the caps. I eventually solved
this problem by changing the current limiting inductor. A safety
gap as I described should not be firing with the 15,000 volt pig
transformer that I was using.
Ed Sonderman