[Arran]

This is interesting, they actually designed the B+ side with some form of voltage regulation, I think? Most of the B+ supplies I have seen, vintage and home brew models, are lucky to have a inductive choke between the input and output filter caps, often just a 1-1.5 K resistor, yet they do not produce much hum if you run a 1940s battery set from one. I always thought that the C- and A+ supplies were much more important for filtering and voltage regulation if you wanted to keep hum out of the circuit, but AES went cheap relying on a single filter cap and a lone Zener diode. I can't figure out why they made the B+ supply so much more elaborate, if you look at the power supply in something like a three way portable like a Zenith T.O, it's just a pi filter arrangement, with maybe a 100 uf cap ahead of the filament string. However Mike/Morzh is the right one to ask, power supply design is how he pays his bills.
Regards
Arran