Hey guys, hope everyone had a good holiday season.

Spent a little time working on my circuits and making sure everything was right. I thought I had everything working and wired correctly, but when I turn up my variac voltage, my KBU1510 bridge rectifier keeps failing. After checking my circuit, I found that my 555 circuit has also stopped working, just outputs a fixed 12v. I haven't messed with this at all and it was working before I started using my variac. I replaced the 555 and got the same result. I checked the component values with my multimeter and everything seems to have correct values. Any suggestions to get it working again?

Thanks for the suggestions Fumeaux and Mads,

-I connected the transistors to the GDT and still got the ringing on the GDT's output (see 1st attachment).
-When probing the timer output with UCC's connected, the waveform is still a good square wave( see 2nd attachment).
-Attachment 3 & 4 are my timer and gate drive circuits, and my complete setup for testing.
-I'm following the final schematic: http://www.stevehv.4hv.org/SSTC5/miniSSTCfnlsch.JPG
-For power, im using a 12v 5a adapter. I kept the two 1000uF capacitors in the supply, but omitted the 2200, as it didnt seem necessary without the rectified 20v. The power supply is smooth and seems to be working fine.
-The only difference in components used was some 1n4004 diodes instead of 1n4007 in the 555 schematic.
-I could not find any shorts in the circuit, and changed out the ICs, but sadly that didnt change much in the GDT output.

-The Hirel GDT doesnt list AL value, but gives an operating range of 50-250 khz. I did purchase a toroid from Digikey (blue core in my first post) with a AL of 10000 +- 30%, however i got similar results from that core. It was an EPCOS B64290L0618X038.
-Ive been worried the DSO may not be giving the true waveform, as it has a max frequency of 200khz. Hopefully my school will have a nicer one i can use.

-Ive also attached the waveform (attachment 5) while probing the drain to source of one of the transistors, and got a square wave output. However I am worried that the odd waveform from the GDT will cause overheating in the mosfets when I increase the input voltage. With rectified and smootch 20vdc the transistors went from 71f to above 125f.