I fired up the filaments on the two 813 Beam Pentodes that I scored. They both work. One 813 is RCA, the other is Radiotron. I'm going to have to track down a microwave transformer to feed the plate on this thing. It wants ~2kV. Will have to rethink a project now, because the 304T is dead. I'm pretty sure people have built amps with 813s.