Following the flow of electrons, the starting point is when the center tap of the rectifier
high voltage winding of the power transformer is negative, with respect to the end attached
to one of the plates. That plate is now positive.
That center tap is connected to ground in most radios. (chassis)
Consider another tube, the output tube, for example a 6V6. Electrons flow from the ground
up to the cathode of the 6V6, where they replenish, those produced by the hot surface.
Those electrons flow up to the plate of the 6V6, on through the output transformer, back to
hot filament of the 80 rectifier.
Those electrons replenish those produced by hot surface of the 80 filament.
The plate of the 80, now positive, pulls the electron back through the power transformer
winding back to the starting place.
Electrons are negative.
Positive means there is a lack of electrons.