I started experimenting with MOTs in a 4 MOT stack on a 6 inch coil
last
year. In my setup I wasn't powering them through a variac, just
straight
mains voltage (right at 120v according to my meter). All MOTs had
shunts
and I used a choke type ballast. The MOTs were placed in a plastic
toolbox
and submerged in mineral oil. The first pair was center grounded.
The coil ran well except the oil would get very hot, so hot that if
it were
a little cool outside, steam would pour out. It smelled like burning
plastic and I could see black particles in the oil which came from the
secondary windings on some of the MOTs.
So I did some research (probably here on the list archives) and found
that I could reduce the heat by taking the shunts out.
I purchased some more MOTs (all the same brand and model as before)
and
built a 6 MOT stack, this time removing the shunts. I placed them in
a
larger plastic toolbox and submerged them with oil. The first pair is
center tap grounded and I made a safety gap on the HV terminals of the
toolbox. No filters and no vacuum to the box.
This link is to my coil running with its old toroid (8x33). Notice
the
primary strikes and my safety gap firing in the background
http://www.youtube.com/watch?v=M9p4h1Md8kc