 Power transformers used in North American vacuum-tube equipment were often color-coded to identify the leads.Black was the primary connection, red secondary for the B+ (plate voltage), red with a yellow tracer was the center tap for the B+ full-wave rectifier winding, green or brown was the heater voltage for all tubes, yellow was the filament voltage for the rectifier tube (often a different voltage than other tube heaters).Two wires of each color were provided for each circuit, and phasing was not identified by the color code.