Amplifiers contain voltages high enough to Kill you even after the amp has been powered Off !!! If you don't have enough electronic experience to identify and discharge the Filter Capacitors which contain these Deadly voltages, take your amp to a Qualified tech. If you don't know a qualified tech in your area, post a request in the alt.guitar.amps newsgroup, the folks there will be happy to help you.
1. Wire a .022 25 volt ceramic disk cap on to a mini toggle switch (on-off).
2. Ground the switch and run the hot wire of the switch to the "In" (or "wiper") of the Presence pot.