[Arran]

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.

Are you sure that you don't have that the other way around? More often then not RCA sets I have run across seem to have used the 5Y4, I haven't see the 5Y3 used in very many pre war sets of any brand but I can only comment on the Canadian produced sets. The CGE and Canadian Westinghouse sets also used 5Y4s, although I have a Westinghouse set that used a 5Z4. There is a type up here called a 2X3 that works out to exactly 1/2 of a 5Y3 in pin out and electrically, but this was a Rogers tube only used in Rogers, Canadian Majestic, and Deforest Crosley sets from about 1937-39. Rogers used a pair of these 2X3s in most sets during that period, before that they used 80s and in 1940 they went back to using 80s.
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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
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The same conversion with the jumpers also works for substituting a 5Z4 with a 5Y4 or 5Y3, dig out the tube manual first. I wasn't aware that the 38-690 used a rectifier tube on top of the power transformer, I figured that with two chassis they would have had the space to install a normal chassis mounted socket.
Best Regards
Arran