Mr. Detrola wrote:
Certainly by that time Zenith was well aware of their serious engineering mistake on the 1940 models, as many power transformers had failed within several months of the radio being sold to a customer. My parents had a 12-tube 1940 Zenith and the transformer burned up shortly after the 90 day warranty expired. Makes sense that Zenith tried to modify the design, but you will also notice that they started changing over to 5 volt rectifiers in some sets. Tying the rectifier cathode to the heater winding only solves the problem of H-K shorts, and does nothing to address P-K or P-P shorts which were also happening in various designs of 6X5 tubes.
By 1943 Zenith had published a "fix" for the problem, which advised dealers to install common dial lamps in each HV lead going to the rectifier socket to act as fuses, claiming they had never had a transformer fail a second time in a set with the lamp/fuses added.
Here's the factory bulletin, the info on the 6X5 is in the right hand column near the bottom. ARF member Ed Engelken posted the bulletin originally. http://www.gvtc.com/~edengel/ZenithWW-II.jpg
The above mentioned link to the Zenith factory bulletin no longer works since my ISP discontinued hosting it. I have attached the bulletin to this message to keep it available on ARF.