Getting back to my P-Rails! I've finally tracked down at Seymour's web site, SOME useful information, BUT NOT ALL, about these pickups.
https://www.seymourduncan.com/products/dimensionpages/p_rails_specs.shtml
Bridge P-90 coil has 10.2 k ohms DC resistance, and a resonant frequency of 3.1 kHz.
Bridge Rail coil has 8.56 k ohms DC resistance, and a resonant frequency of 6.0 kHz.
Now they didn't give the inductance explicitly, so I tweaked the inductance until it gave a resonant peak at about the right place in my circuit. I've asked Duncan support to tell me what the inductance is for the P-Rails (they do list this info for other pickups). I'll update this post if and when they tell me.
Here's the schematic and response of the P-90 coil on its own, with a 500k volume and 100 pF cable capacitance (I left the tone control off).
Peak is just above 2 kHz.
Now, here's the schematic and response of the rail coil on its own. The values are a bit off, as the resonant peak is not in the right place, but the main point of this exercise is that one coil's resonance is "high" and the other coil's resonance is "low". If they happen to be the same, then this doesn't turn out to be a very interesting exercise!
Peak is about 5 kHz.
If you just short these together (parallel humbucker), you get a single strong peak located between the peaks of the two pickups on their own. The peak is closer to the peak of the coil that has lower DC resistance.
Peak is just below 4 kHz.
If, instead, you add a blend pot between them, then you get a range of responses. At one end, it's "more like" the P-90 coil on its own, and at the other end, it's "more like" the Rail coil on its own. However, with the blend pot in place, one coil affects the other even when the pot is all the way to one end! So a blend pot is for sure going to affect your tone and make your passive pickups less distinctive than they are when not connected this way.
In between, it is nothing like just switching the pickups together. One peak goes down while the other one comes up. When the pot is in the middle, you get two peaks. The peaks are also not as high and defined as they are without the blend pot.
If you had active pickups, that would be a completely different story. Here I have added a generic op-amp component configured as a non-inverting unity gain buffer on the output of each pickup before the signal hits the blend knob.
Now you can see that at each end, the distinctive peak of the individual pickups is preserved. In the middle you get one peak going down while the other one goes up and a notch between them.
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My conclusion? A pickup blend knob as described would work best with active pickups.
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LTSPICE - pickup blend pots
