June 23, 2015 13Audio Western Electric 91a Thanks to “PF” in Switzerland the mystery from Read here Is solved….. The field coil lodged inside the 509a cabinet is connected at j2 and acts as choke and field coil. Thanks…. Share this:Click to share on Facebook (Opens in new window)Click to share on Pinterest (Opens in new window)MoreClick to share on Twitter (Opens in new window)