You've got five common alnico magnets, which have more color & character than ceramics. Swapping is easy & there's plenty of threads on that, plus online video. It's a 10 minute process (including loosening strings & retuning)and most magnets cost $5. A few dollars & a few minutes gets you 'new' PU. An example is Duncan's Custom (ceramic), Custom Custom (A2), and C5 (A5); all the same PU, with just the magnet changed. Great thing is, you can do the same thing yourself to any HB.
Because each type of magnet is made of a different mix of metals, the magnet field varies and they 'see' different parts of the string vibration, which translates to different EQ and output. The reason all this is done, is because no two pieces of wood are identical; different grain, density, mineral content, water content, etc. That means you can put the same PU in 5 different guitars and it may sound different in each. That can be very frustrating, but magnets allow you to compensate for your wood's natural tone, and can often get you where you want to be, although they can't work miracles. Here's the cast of characters:
A2 is warm, lots of mids, not much treble, low output. Low end can be flabby in some guitars, especially in a neck PU. It's famous for having lots of character. Good for warming a bridge PU (which is normally bright and sharp); also works in a bright neck PU (like many PAF's). If an A5 PU is too bright, an A2 can take care of that. A2's are being used to tame the infamous 'ice pick' spike of the JB which is prominent in some guitars. Lots of members here love A2's.
A3 is similar to an A2, but with more treble. Comes in handy when you need to brighten an A2 PU. If you have an A2 neck PAF (PG, '57 Classic, BB, 490R, etc) and want more clarity & definition, try an A3.
A4 has a balanced EQ, so it doesn't push sections of the EQ spectrum like other magnets. This can be very handy when an A2 is too dark, and an A5 is too bright. Because it's mild-mannered and subtle, some players consider it bland, others love it. Medium output. Good for bridge & neck.
A5 is the brighest alnico, with a lot of treble & low end, but not as much mids, so it's considered scooped. Low end is typically tight and strong. Gives needed treble to the neck position, although the low end can be boomy in some PU/guitar situations. Can clean up any mud that might be present in a neck A2 HB. Also good for the bridge, to give bite and cut. In the bridge, some players use 250K pots with A5 magnets to shave off some of the treble. Output is on the strong side. Because of it's good output & being well-suited for both PU positions, it's become the most popular alnico today. That's not to say other magnets can't do some things better though.
A8 is high output and warm. Lots of midrange punch, but also has a moderate amount of treble (more than an A2). Not really suited for the neck slot, but can be very good on the bridge. Provides a rich, full tone, and has plenty of output to balance with a strong neck PU. There's a wildcat here gaining popularity, the "C8", which is a C5 with an A8 (a C5 is somewhat scooped, and the C8 has more mids and less treble). A8's are also being tried in low output bridge PU's (PG, '59B, etc).
HB's have one bar magnet. Things really get interesting when you start experimenting with P-90's and HB-sized P-90's,as they each have two bar magnets. You can mix pairs of two different magnet types & really dial in your EQ.
Excited yet?
