Blues 90 pickups are based on Gibson P90 single coil pickups - continuing the same soap-bar appearance, but with a substantially modified interior. Blues 90s have -
Alnico 5 magnets for pole pieces - rather than the steel or ceramic magnets (depending on which "authority" you are reading) used in many forms of P90 pickup (Alnico is a magnetic material made typically with Aluminium, Nickel and Cobolt - hence the name. Alnico 5 is a specific grade.)
windings of 42 gauge, enameled, copper wire (with reportedly fewer windings than P90s)
non-adjustable pole pieces which are all the same height - ie non-staggered and without individual screws
Blues 90 pickup
As far as I can tell, the same pickup design is used in both the neck and bridge positions - although I have noticed recently that my neck pickup has R (for Rhythm?) marked on the base-plate and the bridge pickup is marked with a T (for Treble?). Does anyone out there have certain information about this? Update - see Pickup orientation below
The pickups seem to be wax potted - traces of wax can be found on the pickup bobbins. Wax potting helps reduce pickup microphony (responding to audio signals, as opposed to changes in the magnetic field), which in turn reduces squeal (as opposed to more desirable forms of feedback - which occurs via string/body resonance).
The pickups are mounted direct to the body via two gold-plated screws between pole-pieces 2 & 3 and 4 & 5. These screws allow the height of the pickups to be adjusted - the pickups are supported on springs threaded on to the mounting screws. The mounting screws fit into threaded metal bushes that are set into the body.
The dummy-coil appears to be a Blues 90 pickup without the magnetic cores - which makes sense from an electronic point of view!
Pickup orientation - the bridge pickup has its north magnetic pole upwards/towards the strings, and the neck pickup has its south magnetic pole upwards/towards the strings. This helps cancel hum when the pickups are selected together.
