I have a 58 Zenith Space Command that uses a 5AU4 instead of the pretty much standard at the time 5U4. I may be wrong, but it seems like the specs were almost identical and I can sub the 5U4 in with no problems. This particular chassis design was used for a number of years and through the run they used both tubes. A power supply/rectifier is a pretty basic and straight forward circuit that you wouldn't think would require much design change.

I seem to recall reading that the 5AU4 was the improved higher current version of the 5U4, and that putting a 5U4 in place of a factory specified 5AU4 can shorten the life of the 5U4(but this could be me mixing things up).

There was a lot of things that Zenith did, that didn't seem to make sense. The 58 model was the first of that series of chassis. They regained the favour of the service community, replacing those hard to work on 56 and 57 upright chassis.

I wish that I could locate a good working chassis to throw in my 58-9 (?) 24" Space Command! Anybody got anything from a part out?

5AU4 = 5V3. A 5U4GB in its place may indeed have a short life and would probably cause the B+ to sag even while it was still "good". Conversely a 5AU4 subbed for a 5U4 type might shorten the life of the power transformer because it draws 3/4 amp more filament current.