Wiardgroup

I'm not doc. I don't even play him on tv. However, I have often created feedback loops for fun and profit:

source into input 1 of a mixer
mixer out into the borg (or whatever filter you want)
borg out to a multiple
multiple out 1 to a vca (*)
vca out back to input 2 of the mixer
any of the other outs of the multiple can be used to tap off the feedback affected signal (e.g. to your volume vca)

(*) this is the crucial step that allows voltage control of resonance. Note that you can put anything you like here in in order to change the nature of the feedback - other filters, distortion units, etc. Res on the borg should be set to 0 for the most predictable results.

Also, you don't have to use a filter - this works well with digital delays, too (set feedback in the delay to zero)