The pickups are wound with #42 plain enamel wire. The bobbin wire was purple in the original PAFs, but reddish in later PAFs and patent number pickups. Gibson eventually switched to polyurethane-coated wire around 1966. However, plain enamel wire was used in black dual leads until 1965. With the wire change around 1966, the wire leads also changed to black and white (although there were also humbucking pickups wound again with plain enamel wire in 1967, but the leads were white/black). When the wire coating changes, the sound of the pickup also changes, affecting the PAF's responsiveness. The amount of wire (and coating) wound on each bobbin determines the resistance of the pickup. If more wire than the nominal amount is wound on the bobbin (intentionally or accidentally), the mid-range becomes thicker and the high-range becomes less pronounced. Due to human factors and the wide tolerances of the manual pickup winding machines Gibson used from 1956 to 1961, the resistance value of PAF pickups is typically between 7.5 kΩ and 9.0 kΩ ohms. By 1962 (the end of the PAF era), Gibson was producing pickups with wire that was very consistently 8.5 kΩ ohms (with a tolerance of ±0.25 kΩ ohms). First-generation PAFs (1957/1958) tend to be around 7.5 kΩ to 8 kΩ, while 1959 PAFs tend to be a little "hotter" with more wire. Individual PAF bobbins can show very different measurements due to Gibson's manufacturing techniques. For example, one bobbin might have a resistance of 3.5 kΩ and the other 4.5 kΩ (for a total of 8 kΩ). This mismatched resistance is actually a good thing. Because the resistance of both bobbins is different, certain frequencies stand out. This is why the sound of two PAF pickups can be quite different.