Why are pole pieces used in guitar and bass pickups adjustable and some non-adjustable?
Adjustable pole pieces are used to help balance the output of each string. Fixed pole pieces as used in many Fender style pickups may have rod magnet pole pieces cut to the desired length for better balance and output of each string. Early Stratocaster had pole pieces cut to a desired length for string made back then. Early electric guitars used strings with a wound 3rd or G string. Today many players use a plain G for better bending. The staggered height of the pole pieces gives the Stratocaster a particular sound because the magnets are cut to different lengths. By changing the length of each pole piece will give a pickup a particular sound. The strength and length of each pole piece will change the output and tone of a pickup. Gibson humbuckers were originally designed without adjustable pole pieces and since Gibson P-90 s (single coil pickups often called Soapbars) had adjustable pole pieces, Gibson marketing felt that having adjustable pole pieces on the humbuckers was a good sell
