Question SSTC topload; it arcs in the wrong spots

[thaumatichthys]

Hi, I have been trying to build an SSTC recently, mostly based on Loneoceans' SSTC 2. When I test it with mains power though, it seems to either just produce arcs between the primary and secondary, or between different parts of the secondary. I thought this was because of excessively high coupling, so I redesigned my primary coil to be much lower, which solved the primary-secondary arcing issue, but the sparks that came out of the breakout point were less than an inch long (running on a half bridge with 340V bus voltage, so +-170V). I thought I had made the coupling too small now, so I raised the primary by around 20mm, and now it goes back to sparking between the primary and secondary.

Thinking that the topload wasn't connected well enough, I tried removing the topload, and just using a piece of wire as the "topload" and breakout point, and now it sometimes arcs between the top of the secondary (not the breakout) and around 3/4 down the secondary, and other times it just arcs between the primary and the base of the secondary.

How do people normally attach their secondaries to the topload? i.e, solder? foil tape?

I'm not sure how useful the schematics will be, but here they are
ZD1-4 and C11 on the schematic with the IGBTs are not installed, and on the driver board, I had to change the power transformer outputs to be paralleled instead of in series, since it kept overheating ZD5 (I used a 1N4744 instead).
(In the JavaTC saved coil file, this is before I raised the primary by 20mm)

[thaumatichthys]

I figured it out, it was simply that the current transformer polarity was wrong. I didn't know that it would still oscillate (kinda) when this was wrong.

[davekni]

I figured it out, it was simply that the current transformer polarity was wrong. I didn't know that it would still oscillate (kinda) when this was wrong.

Glad you got it working!  Yes, sometimes there is enough delay through driver and IGBTs that it can oscillate at high frequency, usually a second mode of secondary coil where voltage maximum is near center rather than at the top.  This high frequency often kills IGBTs.  Glad yours survived!