If you have a old power supply with a large choke. You can unhook the two wires that go to the choke and hook your field coil wires where the choke leads used to be.
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You may find what you are looking for at one of the two following places.
This is Angela Instruments. Down the page I linked you to is this item:
J-7. Jensen '45 Technical Monograph about Horn Type Loud Speakers, 15 pages with illustrations. Depicts some classic Jensen horn speakers. $8.
An email or telephone call should let you know if what you want is in that old brochure.
Next, call Vintage tube, Andy Bouwman. He is an expert on field coil speakers and regularly sends out antique literature (Xerox's) in with his tube prices. Bet he has an old picture of this Jensen item as well.