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. Vintage Guitar Article "how input jacks work" Ok, more on the input jacks. The second jack has it’s switch soldered to the first jacks hot lead. Each jack has a 68k (usually) resistor from its hot lead, and they join together to make one lead to the tube stage. These “series resistors” are sort of input buffers – to keep the amp from getting high level spikes from the instruments.