"I've heard that there was a rectifier tube that was failure prone, with heater-cathode shorts. Think it was the 6X5, but I'm not sure if that was the one. "
I think that is the case... I have read of numerous 6X5 failures (mostly shorts). Restorers seem to favor the "X Plate" design.
http://www.antiqueradios.com/forums/...64698e2acbeb0c
jr