Every project starts with a single step and I have been thinking about this project for a while. The vacuum tubes / radio valves in this project have 2 volt filaments, although alternate tubes/valves can be used that have 1.5 volt filaments. The 2 volt valves were "early" valves designed for lead-acid accumulators and the 1.5 volt were "later" valves invented around the 1940s for dry batteries. So I decided to order two "old-style" 1.5 volt batteries to cover both scenarios - if I use the 2 volt valves I will put the batteries in series and use a resistor to drop the voltage to 2 volts.

I think these are called No.6 batteries because they are 6 inches tall:

You can read more about them here: http://prc68.com/I/No6.shtml

They are also used in electric clocks and I bought these from https://www.kensclockclinic.com/our-no-6-batteries/

It's perfectly OK to use 1.5 volt D cells and buying these No.6 batteries was a bit extravagant. But they look suitably retro!