They give you simple Class-A operation from any amp. They not only rearrange the pin locations of the tubes, but also provide the necessary current limiting on the screens and cathode as well as blocking the amplifier's grid bias voltage, while configuring the EL84 in a Class-A, self-adjusting cathode-bias circuit. In other words, there are no adjustments to make and no modifications necessary. You simply plug the Yellow Jackets™ into the amp's output tube sockets, (in cathode-bias amplifiers) screw the ground wires under one of the output tube socket mounting screws, plug the provided EL84's into the Yellow Jackets™, turn on the amp and play.