I will be glad to add what I can. Altec compression drivers, both large and small format, are relatively easy to convert to field coil service because the original Lansing f.c. versions were modified minimally for permanent magnet duty.
The original versions featured an inner pole piece whose diameter was maintained down to the front (exit) plate. Later p.m. versions featured a widened skirt below the voice coil area to conform to the magnet's outer diameter. Most modern f.c. adaptations make the critical mistake of retaining the skirt, leaving inadequate cross sectional area for the field coil. This results in the oft-heard complaints of overheating field coils, low flux density and poor sound. Properly done the results are quite different.
As to the "are field coils any good?" argument, all I can say is that I am convinced. A late 1930s Lansing Iconic two way system simply SLAYS a late 1940s Lansing Sound (JBL) permanent magnet two way system for overall naturalness of sound reproduction. As I told an temporary owner of an Iconic several years ago... "If you are certain that you want to sell it, then don't ever listen to it."
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