A little web searching pulled up this thread describing exactly the same problem and some solutions:
http://www.antiqueradios.com/forums/...p?f=3&t=108670
I put my scope probe near the yoke and found really pronounced ringing.
I flipped though some 21B1 schematics and found the last revisions which did include that filter as part of normal production. No values given though so I'll have to experiment.