Iain M Churches wrote:

Question: How much voltage does the tube rectifier drop,
and how much extra should I allow on the transformer
secondary to compensate for this?

Tube rectifier filaments eat a fair amount of power, and
usually need a separate winding on the transformer. Don't
forget that when figuring how big a transformer to buy.

Figure that a tube will lose about 50V. For a full wave
centertapped transformer that means add about 100V for the
entire secondary winding voltage. to get 425VDC you'd need
about 1200V centertapped.