I'll copy a comment about the DIY thread I made yesterday on the High Efficiency Speaker Asylum:

Thank you for the interesting thread link, Deon.

I won't get into the merits of field coil conversion except to say that IMO the results are worthwhile and not subtle. One thing I'll mention is that a mistake is often made when converting Altec or JBL compression drivers to field coil, and I see it happening again in this thread. The original Lansing 1930s field coil compression drivers used a constant diameter on the center pole from the voice coil gap to the back (actually front) plate. A sectional view can be seen at the bottom of this page:

http://www.lansingheritage.org/image...943/page07.jpg

When the conversion to Alnico V magnet was made in 1944, the center pole was shortened so that it could sit atop the interior ring magnet. Also, a skirt was added below the voice coil gap to widen the center pole to the diameter of the top of the magnet. This occurred with both small and large format drivers, as can be seen in the drawing of a small format driver here:

http://www.lansingheritage.org/image...804a/page1.jpg

Most folks who convert these drivers to field coil have never seen an original Lansing apart, so they machine a new lower center pole section to replace the magnet using the incorrect wide diameter of the skirt. Then a rather anemic field coil is wound to fit over this wide diameter, which will have inadequate cross section resulting in low gap flux density and/or possible coil overheating. When making the conversion, the skirt should be machined off the existing phasing plug/center pole and a new section of the same diameter machined to replace the magnet. Then a field coil of correct dimensions can be wound and the motor will work as well as the priceless 1930s Lansings.