[martinj]

How does the  5W4 and 83 tube filaments get fed?

[DaleHCook]

Note that no power cord, power switch, or power transformer switch are shown. That (along with my experience with full schematics for other Hickok models) would indicate that the power transformer has individual filament secondary windings (which are shown) for those tubes

[RodB]

Absolutely no one is going to reverse engineer that circuit. Even the iron core is missing.

[morzh]

I think they have only shown the secondaries of the transformer.
Two of them feed the rectifiers' filaments.