[Doug Houston]

The issue of 5X4 and 5U4 is simple. The high current rectifier, 5Z3 was developed about 1933 or 1934. It amounts to a beefed up 80, with the same 4 pin base. You will find it in some Philco sets, such as the 680, and earlier, the console model 16. It was an RCA tube, and before octal based tubes were made, it was the only high-current receiving rectifier.

With the use of octal based tubes, Philco and Zenith, RCA's fiercest competetors, needed and octal-based 5Z3. Sylvania was the supplier to Philco and Zenith, so they simply put an octal base on a 5Z3, and called it: 5X4. Now, RCA had not as yet, put a new base on the 5Z3, and was not about to use a Sylvania design, so a couple of years later, they finally got around to re-basing the 5Z3. They did so, used a different base connection, and called it 5U4. They did it in ime for Television, and high current power supply applications, and as a result, RCA sold far more 5U4's than Philco and Zenith used. Since the industry was swamped with 5U4, rather than 5X4, RCA's baby won out. You never heard of the 5X4 in new equipment after 1938. Note also,that Sylvania re-based the 80 and called it 5Y4. RCA did the same thing, and called it 5Y3.

Now, if you'd want to make the set use either 5Y3 or 5Y4, or use either 5U4 and 5X4, the socket can be wired for the base connections of both 5Y4/5Y3 or 5X4/5U4. I haven't a tube chart handy here, but a quick look at the two base connections will tell you which pins to connect together, to have the socket take either tube. I've done it several times

On the issue of the 14 inch speaker, it may be possible to simply use a PM dynamic speaker on the set if the load of the field on the power supply is not critical. That's something that you'd need to research. I've never investigated it, but I wonder if all of Philco's type "W" speakers have the same field winding?
I had seen one or two on eBay, so hope still springs for you. Remember that, when the big Philcos were being built, the strong magnetic materials were not yet developed. There were Alnico magnets, but not Alnico 5, until just in time for WW II. That's why a lot of speakers with high resistance EM fields were used prior to 1942.