The 6X5 problem was especially bad in Zenith radios, known for having somewhat puny power transformers — no matter what other virtues their sets may have had.
Over a period of time, the 6X5 would first develop a heater to cathode short. As one side of the heater circuit is grounded, that means you will eventually have a B+ short to ground. This can happen so gradually that a line fuse might not blow until it is too late.
In the early 1940’s, Zenith sent a service bulletin to their dealers recommending that a pair of #44 bulbs be wired in series with the B+ leads from the transformer, supposedly saving it in case of 6X5 failure. I have no idea whether or not this worked.
Zenith did send replacement power transformers to dealers who were servicing multi-tube consoles, which put an extra strain on 6X5’s, and these replacements had a 5-volt winding for a decent rectifier. The late production 6X5GT’s had some design improvements, but I still don’t trust them. There is no way I would put a 6X5G in ANYTHING.