Fireworks in an Audio Research VT200

I turned on my amp the other day and as it was cycling through the start up process, I heard a large pop and then an enormous fireball flew out of the top of it. I immediately unplugged the amp. After taking all the tubes out and visually inspecting everything, I concluded that no tubes were blown. I put it all back together and it sounds great and I have had no problems since. The bias is fine.

Has anybody had experience with this sort of thing?