All of the posts regarding the location are correct, but these pictures should clear it up nicely.
http://i78.photobucket.com/albums/j100/nolanwoodbury/K731C.jpg
Note that the wire connects to the tuning cap, then runs a separate wire that loops through a hole in the bottom of the chassis that connects to (I believe) pin 7 of the 12BE6. Not sure of this, 'cause the markings are worn and this radio has no tube identifier-label on the bottom. Still, it's the tube closest to the tuning mechanism.
http://i78.photobucket.com/albums/j100/nolanwoodbury/K731E.jpg