Field Coil Jensen Speaker 1928

Hi everybody,

I have a very old Jensen electrodynamic speaker :

I found it in a RCA loudspeaker 106 (maybe modified) :

The tube amp uses 6005 (6AQ5W) and EF40.

here's the diagram :

The problem is : I don't know what wiring I need to use on the field coil of the speaker, neither the voltage I should use on it (100V DC ???)

Thank you for your consideration.