Adding an expression pedal to an MS20 filter

[Taylor]

I built Tim Escobedo's MS20 clone. I tried to add an expression jack, by grounding the sleeve of the exp. pedal and connecting the tip and ring to the equivalent lugs of the frequency pot. Powered it on, smelled something nasty, thought it was my tube amp buying the farm. Decided it wasn't, went to adjust the filter frequency and burned my finger - turns out both the freq. pot and the pot inside my espression controller were cooking.

Why did this happen? How can I do this without cooking anything further? Could I just hook up the expression pot using only 2 lugs as a variable resistor between 9v and the circuit?