Op Amp Muff Cap Issue

[bigzed900]

Hi all,

Can anyone explain what the 1.uF Cap does connected to pin 3 Op Amp 1 (4558)? It causes feedback when the tone and sustain are increased to max. I removed it and it seems to work ok...so can I leave it out?
 
Cheers
Bill

http://www.pisotones.com/BigMuffPi/psst/schm-opamp.gif