[mikethedruid]

Early PCBs did not mix well with vacuum tubes because the heat from the tubes ended up causing problems with the boards. Even when they used special long lead heat isolating sockets this did not fully solve the problem. It was not until things switched to transistor circuitry, and improved PC boards that this became less of a problem. Early PC boards were usually one sided, and used jumpers on the blank side where needed. The traces were kept as large as possible by convention to lessen the chance of the tracks loosening from the board itself. One basically just removed enough copper between the tracks to isolate them.. How do I know? PC board design was one of my specialties when I was working. I went from black puppets and traces on the mylar sheets on a light board, for one sided boards, to black puppets with red and blue trace tape on the mylar on the light board for 2 sided boards, to CAD on a PC board and 4 layer boards.