[morzh]

If it is functional it is fine then, weak or not. Unless dead.

Here's how it is:

The unusually high voltage across the first filter cap #56 can be due to mostly the lack of load.

This could happen if your field coil is open, or if the resistor 57 is open.

But all your tubes have plate voltage relative to the Chassis which means neither of the above are the case.

So, also if the tubes do not draw current at all or very little. But if all tubes glow and all test OK (even if a bit weak) again, this is likely not the case.

See if there are oxidized pins on tubes or sockets, especially on the 42. It takes for the 42 to complete the circuit meaning that the Cathode and Anode have to make contact.