I saw some tubes today that were marked (I think) 676 Power tubes, and I was wondering if anyone knew what they were and if they would be good to build a tube TC. They were power company surplus, were about 8" or so tall, had 4 pins on the bottom, and a connector on the top of the tube. A paper in the box warned that xrays would be produced if the tubes were used above 15kV. Any input? Thanks, Steve Roys.