[Mondial]

Skip, make sure you are measuring the transformer voltage directly at the transformer leads. If you are connecting the voltmeter probes after the clip leads or ammeter, you will see a large voltage drop as the leads have significant resistance. You could easily see a drop of 1 volt when passing 8 or 10 amps through a typical clip lead.

With a 2.5 v supply it is very critical to have a minimum of resistance in the connections and wiring, which is one reason 2.5 volt filaments were abandoned in favor of 6.3 v tubes in later sets.