[Arran]

 That Majestic looks like it may be around 1932-33 vintage, so not quite at the end of the line for Grigsby-Grunow, believe it or not they actually did live long enough to switch over to using 6.3 volt tubes. Other then the pair of 4-S tubes, which look like they are double diodes, most of them are just metal spray shield versions of the second generation 2.5 volt type tubes as denoted by things like 58S or 2A7S.

http://www.nostalgiaair.org/pagesbymodel...008791.pdf

I'm not sure why they would have used two of those 4-S tubes, they use one diode in one tube for the AVC diode, and one diode in the other for a second detector, the plate on the second diode inside each tube is grounded. Although unlike something like a 6H6 both diodes in each tube share a common cathode, still this didn't stop manufacturers from using both diodes in a 75/6Q7. What I am wondering about is what could be used as a substitute for the 4-S tubes in on of these? A type 56 wired as a diode? I have a Sylvania tube manual from 1937 that has a section on Majestic tubes so I will have to see what the specs are and what may be subbed for one.
Regards
Arran