Input Jacks - Rob Robinette's Car and Tube Amp Website Common Input Jack Wiring. This is how many people actually wire the four input jacks. The jacks are oriented so the upper jacks' Tip terminals and lower jacks' Switch (shunt) terminals are touching and they are soldered together. Doing this deletes the jumper between the two jacks. How to Fix the Input Jack on a Bass Guitar | Our Pastimes The so-called "input" jack of a guitar is actually an output jack that sends the signal from the pickups out to the amplifier. Over time, the jack itself can become loose or the wires that are connected to it can become loose as well.