[MrFixr55]

Hi Antipodal,

Greetings to the "other side of the world! Though I may never get to Australia and New Zeland, I always had interest in your 2 nations.

Yes, the field coil is in series as a filter choke in the B+ Supply, so it did dual duty, without eating much current. Remember, the output of the inverter in this unit is practically a square wave, so the choke was a good way to filter the spikes. There were some 6V radios whose speaker field was directly across the 6V input. These likely drew current.

"PM" magnets for speakers were available prior to WWII but were expensive, heavy and not as strong as the field coil type. However, in battery operated radios, especially those using B Batteries instead of an inverter, there was not much choice other than the use of a permanent magnet in the speaker. In fact, some of the battery powered radios from the 1930s used the older magnetic style speakers, similar in function to the early cone type speakers of the 1920s before the "Kelogg - Rice electrodynamic" speaker, the archetype of all modern moving coil speakers. The invention of the AlNiCo alloy for magnets allowed for a stronger, cheaper permanent magnet. The dry electrolytic capacitor also allowed for larger capacity, thus eliminating the use of a choke (i.e. the field coil) in the power supply.

Good luck with the radio!