An update about this coil: after maybe 20 minutes combined operation time one of the tubes began arcing from the screen grid to ground on the outside surface, and then it arced on the inside a couple times. This was due to two things. I had too many turns on the feedback coil producing too much voltage on the screen grids, and the tube that arced over was actually a tube rated for 100w less plate dissipation than i thought it was. The tube had no markings anywhere on it, but i was able to figure out its probably an amperex or a general electric 4x150a, not the 4x250b i thought it was. That means there was only a combined plate dissipation of 400w here, which makes the 18-20" arcs on the output even more impressive (to me at least.) I will be dissasembling the coil, finding a suitable base to build it on, and remaking it in a much neater fashion and hopefully it will last a lot longer with a bit of modification. I will also be making a mains synced staccato controller to run the coil in pulsed operwtion to hopefully make any failures in the future very momentary and short lived, reducing the chances for permanent damage to these relatively expensive tubes(that i just hapenned to get really good deals on, $100 for six of these kinds of tubes i think is pretty good.)