Tube question

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?


Steve Roys.