Ross Phaser Regen Distortion Help

[Tall Steve]

I'm getting a nasty distortion when the regen pot is turned all the way down.  Changing the pot didn't fix it, and I don't think the pot is bad because it worked fine throughout its rotation when I used it for the rate control.  I subbed in film caps in place of the electrolytics to remove any polarity issues--no luck.  The distortion goes away as soon as I add even the slightest bit of feedback.  Just as a control test, I pulled the 27k going to the regen feed and it worked with no issues.   Any ideas?