[Ron Ramirez]

But I guess I should use (a polarized plug), with B- connected to the wide blade?

Yes!!! Safety first!

It may well be that by using a polarized plug, and ensuring B- is tied to AC neutral and not hot, that then you may be able to once again connect pin 1 of the 12SQ7 to chassis, as my 54 is.

Also please take note of John's (MrFixr) comments - and John, thanks for weighing in here.

John, yes, manufacturers used the AC switch in the neutral side of the line in an effort to reduce hum. They also did not use polarized plugs back when these sets were being made.

I use a polarized cord and plug on any AC/DC set I work on, whether it is modified (like my 54C) or otherwise kept stock. I also change the wiring so the AC switch is in the hot lead instead of the neutral. I have not noticed any increase in hum in the sets I have changed the switch wiring on in this manner.