Yes, It's possible. I had a depth pot on the breadboard for a while. If I remember correct I used a 500K or a 1M pot to ground at the output of the LFO. On my vero layout just connect the lug 3 of the pot to the row under the J201 (row/column T-17) Then connect pot lugs 1&2 to ground. Not sure about the pot numbering so expriment.
Another way to make depth control is to replace the top left 15K resistor with a pot. Use a 20K pot from third row (C-1) to ground. This will also change the filter/crossover frequency as well as the depth of the vibrato which is cool. That method will only effect the vibrato mode though and not the stutter.
EDIT. Another cool mod is to widen the range of the LFO. By replacing the 270K resistor with a 100K. That will get you into superfast psuedo ringmod'ish sounds aswell. And by lowering the 15K resistor (that is next to the 270K r) to a 3.3K you will still have the same slowest speed. A log pot makes a better sweep with that mod.. but I found it a bit too extreme and harder to set an exact tempo within such a wide range.
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