Way back when (yes, I'm getting old, but not that old!) a friend of mine ran a pair grid-driven 4-1000As, with 5,800 volts on the plates. The drive power was almost nil, and the output was well over double the legal limit, even by today's standards. Why on earth anyone would need one with 4 tubes is beyond me. This said, Emtron can't make enough of their 4 tube, 3 phase powered, monster. Operating skill? Who needs it with that kind of output power?