The decades after World War II were a battle for supremacy between the two then-giants of guitar tech: the Gibson Guitar Corporation and the Fender Electric Instrument Company. Unlike in real wars, everyone was a winner.
Gibson had spent WWII making wooden parts for the military; getting back into guitars, the corporation needed to invent something to take some of the growing electric guitar market from Leo Fender. The Gibson Corp. got stuck into R&D. One of ideas which emerged was a prototype for a pickup with six adjustable pole pieces protruding through a black plastic cover, secured to the guitar with dog-ear end lugs. In 1946, this was released as the P-90.
From its size you might think it’s a humbucker: it’s roughly twice the size of a standard Fender pickup. In fact, the P-90 is single-coil—but it’s a big coil. It comes in three main shapes: the “soap bar,” a rectangle in which the mounting screws are positioned between the pole screws (leading some to think that it has eight pole pieces; it doesn’t); the “dog ear,” where the mounting screws are in extensions to the ends of the cover; and a humbucker casing allowing you to retrofit a P-90 to a guitar routed for humbuckers (such as the Les Paul Standard).
Gibson needed something which would compete with Fender’s crisp, clear pickup tone. To grab the market, it needed to be technically as good, but with a different aesthetic. Two main lines in guitar sound evolution date from this tech war: the translucent, wiry tone of the Fender, as opposed to the fat Gibson sound. We can trace the evolution of the sound through the solo on the Beatles’ “Tax Man” — that’s McCartney armed with P-90s — versus the leaner sound of Hendrix’s Fender pickups on Are You Experienced? We can go on, if we dare, to contrast Guns’n’Roses fat lead guitar sound with Eric Clapton’s modern-day crystalline semi-clean blues. Jazz went for Gibson early on; one of the P-90’s older sisters was named after Charlie Christian, a fan of the sound.
Inside the P-90 there’s a large, flat coil with adjustable steel screws as pole pieces, and a pair of flat alnico bar magnets lying under the coil bobbin. Adjusting the pole pieces, screwing them closer to or further away from the magnet, alters the signal strength and tone. The sound is distinct: it has the single-coil clarity and responsiveness, but the sound is thicker than Fender’s crisp and snappy single-coils. It still has some single-coil twang, but with more punch in the midrange. P-90s age gracefully over the decades as the alnico magnets weaken. An older P-90, being slightly demagnetized, has a slightly softened sound; wiser, if you like. Some specialist pickup makers will build you a P-90 with pre-demagnetized magnets: a bit like wearing pre-aged jeans. I say, buy your P-90s now; enjoy in retirement.
In the rush to compete with Fender, Gibson made technical compromises which led to what some consider a drawback. The P-90 is susceptible to the 60 Hz hum induced in its coil by external electro-magnetic fields. All single-coils have this property, but the power of the P-90 makes it worse: having around 2,000 more turns of wire in its coil than Fender single-coils, the P-90 produces a relatively larger amount of hum. You don’t have to see this as a drawback, of course: remember John Peel’s defense of the surface noise of vinyl: “listen mate, life has surface noise”. It goes damn well for hum as well. Gibson didn’t see it this way, however, and by the early 50s was looking to replace the P-90 with Seth Lover’s new humbucker design.