Those of us who read pickup specs will have noticed the various types of magnets – and yes, they all sound different. The most commonly found material is Alnico, which comes in various grades, all designated by Roman numerals.
Alnico comes in grades from I to VIII but II, III, IV and V are most commonly found in pickups, and you can find plenty of opinions on the internet regarding sonic characteristics. Powerful ceramic magnets are also popular amongst rock players who prefer high-output humbuckers with plenty of brightness.
Back in the day, Gibson and Fender had to accept what was available when they placed orders, so vintage pickups were made with a variety of magnets.
You can easily swap the magnets of PAF and P90-style pickups by loosening the backplate and sliding them out (PAFs have one and P90s have two). Use a compass to determine which side is north and ensure the replacement magnet is oriented the same way before you slide it in and re-tighten the plate.
You can try calling pickup manufacturers to see if they’ll sell you magnets, but many will be reluctant. Stew Mac sell Alnico V bars, or alternatively you could contact Sensmag Inc.
7. Wiring Secrets Pt.7