[Arran]

Hi;
  With regard to the original poster's question about battery voltages I thought I should interject. In most battery operated radios of the 1930s, up until the advent of "low current" battery types that debut around 1939 or so, the standard filament voltage was 2 volts, as that was approximately the voltage of one cell of a lead-acid wet cell battery. Do not run 2 volt battery tubes at 2.5 volts, or they will not live too long, they won't instantly burn out, but they won't last long either. I think you could find a 2 volt rechargeable battery, otherwise you could use two 1.5 volt batteries, in series, with a diode to drop it down somewhat, under load it should be pretty close for testing purposes. Building a proper battery eliminator should be the long term plan, there was a reason why they never really made many battery portables prior to the war years, it was not very practical thanks to filament current consumption.
  I don't know whom actually manufactured sets for Gamble-Skogmo, but it may have been Continental Radio & TV (Admiral), however Riders has them listed under Gamble-Skogmo so that's where the service information is to be found.
Regards
Arran