[Radioroslyn]

You are going to need to get your filament voltage up to 4-4.5 volts. Don't think 3 going to work. Got an old car battery??? Could use that with a 5v 5amp regulator (like a LM7805 in a TO3 package) or a LM317K it's adjustable. Back in the late '20's that's what was use to power the filament was a 6v car/ tractor battery. Got an old Farmall or Massey-Ferguson??? DO NOT CONNECT THE 12v CAR BATTERY DIRECTLY TO THE FILAMENT BUSS OR YOU WILL BE LOOKING FOR A NEW SET OF TUBES!!!! As for the tube mounting the one tube that is spring mounted is the detector tube. It's mounted like that to minimize microphics ( a ringing sound when tube is vibrated). The 71A should be inserted into the socket that is connected to the spkr terminal. It's not a deal breaker as the set will work it where ever you put the 71 but will work better if it is in the proper place.
Terry