I had a troll around Ebay to see if I could find some white chicken head knobs on the cheap. Found these from a very nice dealer in China. even with the shipping I ended up with 8 knobs for my AD50 for around $10.00. I may go back and get a set for my AD120.
http://cgi.ebay.com/4Pcs-Chicken-Head-Knob-White-pots-w-6-35-1-4-/270755064729?pt=Vintage_Electronics_R2&hash=item3f0a426b99#ht_539wt_1139

