Seymour Duncan says of this model "Seymour discovered Billy’s bridge pickup had slightly more output than most P.A.F.s, which helped to give it the extra drive that make this pickup so special." This model dates waaaay back to 1979, which also happens to be the year of my introduction, as well. A simpler time when an overwound PAF was "so special".
This pickup appears to use the same components as the Seymour Duncan '59, with plastic spacer and the like.
According the specs, the bridge is hotter than the Seth Lover Bridge by about .33 henries, with a peak resonance that is very close, without a cover. The neck pickups appear to be nearly identical, spec wise. Both sets also use AlNiCo 2 bars, though I measured a higher Gauss at the slug and screw tops of the neck pickup for whatever reason.
Seymour Duncan Pearly Gates Bridge, no cover, 4 cond., mfd 2014 Series resistance: 8.31k ohms Series inductance: 5.097H Gauss: 290G slug, 310G screw , AlNiCo 2
I want to know what's the difference of using A2 vs A5 magnets for PAF / humbucker styles?
What is the effect in the inductance, resonant frequency and voltage output?
For example, given a PAF with A5 and then changing the magnet to A2 or vice versa.
Is the result similar to when the magnets are the poles like Strat single coils?
Since the bar magnet isn't in the core of the coils, it has a much smaller impact on the inductance. Changing from A5 to A2 only increases the inductance about 20mH, or 0.02H. The measured Gauss usually drops from around 300 to 350G for AlNiCo 5, down to around ~250G with AlNiCo 2.
Just considering the facts, the primary effect is that you get less magnetic pull on the strings, and slightly less output. A lot of guitarists on the internet have imaginative descriptions about how A2 and A5 bar differ, as well as rough cast, AlNiCo 3,4 and ceramic magnets, but I think all of that serves to demonstrate that a very minor difference can seem like a very big change when the imagination gets involved, and people exchange their imaginings with other people on the Internet.
I have watched some video of changing a humbucker pickups from A5 to A2, and it seems that there is really a noticeable difference. Where do you think can we attribute that change in tone or response?
How about the case of strat pickups where the poles are the magnets? Is it similar or are we expecting a more pronounced change?
What is the relationship between the magnets permeability and inductance of the coil and its output voltage? I want to study this further. Can you share some links to read?
It's the same difference that you'd get from lowering the pickups, it's just two different ways of weakening the magnetic pull and the flux density at the strings. When string pull is reduced, the strings' vibrations decay more symmetrically, since you don't have a magnetic pull stiffening one half of the string. Magnetic pull promotes higher harmonics by promoting the smaller divisions of the string, just like pinch harmonics, but instead of inducing the harmonics with a hard finger tip, it's just a soft magnetic pull, so it's a lot more subtle.
If the pole pieces are AlNiCo, like a Strat, the pull is a lot stronger, because the AlNiCo itself is a lot stronger that steel pole pieces with some other magnet underneath. And since the magnet is in the core of the coil, it increases the inductance a lot more. In this experiment guitarnuts2.proboards.com/thread/9039/alnico-pole-piece-comparison , the difference from AlNiCo 5 to 2 was almost 120 mH, or about 5% higher overall inductance for the test pickup, which possibly makes for an audible difference, but not a big difference. The flux density at the pole top of A5 in a Strat pickup is around 1050 Gauss for the G and D pole pieces, and only 700 Gauss for A2, so again, less string pull, more symmetrical string movement, more fundamental and less harmonics.
The relationship between inductance and voltage output is a correlation. Inductance and voltage output but increase when there is lower magnetic reluctance (less air and inert material) between the strings and the coil(s). Steel pole pieces and AlNiCo replace air (or plastic) with permeable material, material that supports a magnetic field, which reduces reluctance, therefore increasing the inductance. At the same time, because it increases the "coupling coefficient" between the guitar strings and the coils, more of the "magnetomotive force" of the guitar strings is present in the pickup's coils, and so there is more voltage output. Again, it's the same as if you raise the pickup closer to the strings, you're reducing the amount of air between the strings and the coil, and increasing the coupling coefficient between the strings and the coil(s). This is also the main reason why humbuckers are a lot louder than single coils, all other things being equal, more of the pickup is placed physically closer to the guitar strings.
As far as reading, all of this is very closely related to transformer design, but instead of two coils, you have one coil, and the magnetized guitar strings that act similar to a second coil, so if you read up on transformer design, such as this www.electronics-tutorials.ws/transformer/transformer-construction.html , and you conceptualize the pickup and strings as two halves of a transformers, the principles apply. The only thing to keep in might is that the pickup analogy results in a coupling coefficient that is very low, around .01, where as an ideal transformer would have a coupling of "1", so to the extent that pickups and string are like a trasnformer, it has to be though of as an extremely inefficient transformer, where the two coils are far apart, and there is no core in between them. A guy who goes by "Tele Tucson" actually made a working transformer LTSpice model that matches with experimental measurements very closely, so if you just learn all there is to known about transformers, then a lot of what there is to know about guitar pickups will just suddenly make sense. Not just the coupling, but also concepts such as leakage inductance and stray capacitance as well, which are relevant to guitar pickups.