[morzh]

Let's add here the power transformer. In principle, this is exact same type of the transformer as the Audio Out.

Only it matches the power of the AC source (Mains, 120V in the USA case) to whatever the load type is.
This is why you see a, say, 120V and 1A in the primary (at full load of a tube radio), and then the sum of the powers of A+ and B+ being equal to it.
You simply matching the load: low voltage/high current filaments' load and high voltage/low current plates, SG biases etc, that demand small currents but 300-400V voltages.