[Arran]

Looking at the model 20 schematics, it looks like 45s could be used in the push pull output easily enough by changing the filaments from being in parallel to being in series. The 2.5 V filaments would then add up to the 5V of the winding for them in the power transformer. Whether this was ever done or not, I don't know. It certainly COULD have been a WW2 mod if a repair man had 45 tubes in stock but did not have 71As. More likely someone just stuffed in some tubes to make it look "full." Poses an interesting question though.

   Whilst at first though it seems possible it's very unlikely that they would have tried that. For one thing type #45s and #71As are directly heated tubes, so in order to bias them correctly involves using either a center tapped filament winding, with a resistor going between that center tap and ground, or lacking a center tapped winding, a center tapped resistor across the filament of each tube. Another issue may be the filament current, #71A filaments run at about .25 amps a piece like an #01A, #45 filaments run at 1.5 amps a piece, so even in series configuration that's three times the current draw that the #71As run at. For another during the war years the earlier AC tubes were among the easiest tubes to come by since they had been in production for more then a decade, the hardest tubes to come by were the 150 ma types that were used in AC/DC sets since some types debut just before the war.
Regards
Arran