 Earlier pickups (around 1952) had Alnico 3 magnets, but in 1957 Gibson switched to Alnico 5.One negative aspect of the P-90 pickup is its susceptibility to 50 Hz / 60 Hz mains hum induced in its coil by external electromagnetic fields originating in mains-powered electrical appliances, motors, lighting ballasts and transformers, etc.This susceptibility is common to all single-coil pickup designs, but the P-90, having around 2,000 more turns of wire in its coil than Fender pickups, receives a relatively large amount of mains hum.