I have had enough experience with 6X5’s to be leery of them, but next to no experience with 6X4’s.
I checked several vintage radio sites and discovered the 6X4 also was subject to the same type of failure as the 6X5.
You will find pro and anti camps on 6X rectifiers, usually on the same threads.
The pro 6X camp usually blames Zenith for cheaping-out on transformer design and manufacture. The anti camp will quote sets other than Zenith in which the transformer failed due to a shorted 6X.
You might want to check out https://www.antiqueradios.com/forums/viewtopic.php?f=g&t=66501. This is a thread which started out being a discussion of the methods of cutting the glass envelope on a 6X5G, so the innards could be replaced with a silicon diode, resistor and pilot lamp (or LED) in order to maintain the look of a tube, sort of. It rapidly changed to a sometimes heated discussion of the 6X family of rectifiers.
I quote one poster from that thread:
“6X5’s and 6X4’s are transformer killers, period. I have seen a number of transformer failures in various kinds of gear related to these tubes. They are a cheapskate solution for those manufacturers who did not want to bear the cost of a separate 5 volt winding in their transformers. Even in communications gear which is usually fused, the transformers fry. What happens is that the heater to cathode oxide insulation becomes leaky or intermittent. This increases the current drain on the B+ winding above the rated capability of the transformer. Often, the increase is not sufficient to blow a line fuse, just enough to cook the transformer.
“I invariably pull all such rectifiers from any gear that I get that uses them and substitute silicone diodes. As a result, I have quite a few lying around. I will be happy to send them to anyone who wants them if they will pay shipping costs…just don’t come crying to me when your power transformer melts down…I warned you!”
- This reply was modified 1 year ago by Eric Strasen.