I've got a pair of old (85) Carvin pups with 3 wires and a shield coming out of them. I'm replacing them with 4 conductor Duncan pups. The wire colors of the Carvin pups are Black, Red, White, and shield. I believe the red is hot, and black & sheild are ground. What is the white, compared to Duncan pups? Is the white, the combination of the Duncan red & white?