[MrFixr55]

IMHO, Between the transformer common (where you have one lead) and any point on the rotary switch, you should be reading ever increasing heater voltages (1.5, 2, 2.5, 5, 6, 12, 25, etc.) on either an AC VOM or DVM set for AC. Measuring at the wiped contact (the common for the switch), the voltage cshould match the voltage that the selector switch knob "pointer" points to. Since the transformer should be able to deliver at least 1 Amp at any filament voltage, even a VOM as little as 5KOhm/V should give the same reading as a DVM. A DMM has a very high input impedance, but I too am scratching my head. If a 6V glass tube is inserted, will the tube light?

Thoroughly clean all switches and inspect the various test switches for carbon tracks.

As to how plate voltage is measured, it is more likely plate current. Seems that cathode and control grid pins are tied together to ground, and screen, plate and diode plate (for 75, 85, 2A6, etc.) are tied to the meter with the other meter leg going to the transformer "hot" side. (The transformer primary acts as an "autotransformer) so a value greater than 120VAC is placed between cathode and plate of tube.) This is an emission tester, and all tubes appear to be tested as diodes. I would think that with no tube in a socket, the meter should be at zero unless the short test button is pressed.