[Arran]

Thanx, Arran. (and I think Ron, too).

The 6X5 issue is an interesting one. Zenith was the big offender (player), in the use of 6X5's in the rectifier duty. Our friends in Chicago had a nasty habit of bloating their tube counts, even a little. I'm sure that they considered it innocent enough to put two 6X5's in place of one 5Y3 (or 5Y4) in the rectifier sockets. I've never had a problem with having a 6X5 in an auto set. Zenith owners have had lots of trouble, though. I attribute that to the fact that the "Z" boys ran the 6X5's at higher plate voltages, and caused flashovers that happened lots less in automotive sets.
Go figure: There were loads of automotive sets that used 84 rectifiers. I never heard of ONE of them flashing over in a car set. And, quite a few of those auto sets were PHILCO!! And also remember that the 6X5 is simply an octal based 84! A 6X5 flashing over in a household set, meant curtains for the power transformer, and Zenith had no reputation for over specced power transformers

Back to the 37-650, I'm going to look into the accessibility of the rectifier socket.

I've tried explaining that to people but they just won't believe me, that a 6X5 is an octal based 84 tube, and a 6X4 is a nine pin miniature 6X5. I have also explained that I have owned at least a half dozen small AC sets that used 6X5s and not one had a barbecued power transformer, some even still had the original 6X5 in it as the day it left the factory. If they were such a ticking time bomb I had better buy a lottery ticket because I must have a big prize coming. The reason is that Stewart Warner, Packard Bell, Dominion Electrohome, and presumably Philco and most car radios used them in their proper application, a five or six tube set with a 6K6 audio output. In radios where the maximum current draw was bellow 70 ma and the input voltage was bellow 325 volts AC per plate they would be fine as far as I can tell, most of these were well below both figures.
From what I read the brand Z boys thought it would be a cute idea to power a ten tube set with a pair of them, probably with a pair of 6F6s in the power output stage, but that's what you get with a used car salesman running a radio company.
Regards
Arran

P.S Maybe you could make an adapter if it proves difficult to access the bottom side of that 37-650 rectifier socket? If there is enough room inside the cabinet.