[morzh]

What I cannot understand from your posts is this:

In post #3 you said "No" to the question of continuity across prongs of the power plug when the switch is ON.

Now your switch is working (dirty but OK), your tubes are fine and the pwr cord is OK too.
Unless your wiring inside is bad or your tube panels are badly oxidized you should have that continuity.

Go check EVERYTHING along the filament chain, starting with prongs.
Prong-switch pin 1, switch pin 2 -1st filament pin1, 1st filament pin 2 - 2nd filament pin1 and so on so forth.


PS. The switch typcally can be taken off the potentiometer and cleaned. BTW the pot itself can benefit from some dab of lithium grease and Deoxit mix.
When everything else works.