"motorboating" oscillation sound on a Tonebender Mark II

[mordechai]

I built a Tonebender Mark II circuit with a silicon PNP at Q2 that sounds great.  But I incorporated a 10KB pot between the battery and the -9V in to the circuit to follow the "dyning battery simulator" mod that I saw on the Beavis site.  When I turn the pot to reduce the flow current going into the circuit, I get a really bad "motor-boat" sort of oscillating sound.  Is there a way I can get rid of this?  Use a different value pot?  Swap out a different value Q2 (maybe high or lower gain?  It's currently around 90 Hfe). 

[PRR]

If you don't want instability, the sag resistor must go on this side of the main filter cap: