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P-90 Construction?


 
11/14/2004 1:55 PM
John Culp
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P-90 Construction?
Anybody know where there's a cutaway pic of a P-90?  
 
Or can you just explain to me how it's laid out? I saw on one site Alnico 2 magnets for P-90s that look much like humbucker magnets. Is the magnet turned edge-up inside the bobbin, or is it one with the poles on the flat sides under the coil with an iron pole piece going up through it? The screws under the strings, are they threaded into holes in a pole piece, in the magnet, or what?  
 
Thanks! There's just not much technical information that I've been able to find online about P-90s.  
:)
 
11/14/2004 7:47 PM
John Culp
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Well, I've answered my own question about the basic layout, at least.  
 
First from the diagram of a P-90's magnetic field at: http://mywebpages.comcast.net/skgs/sk/Images/pickups/Pickup%20stuff/Magnetics.htm  
 
Then I found a good drawing of the basic P-90 layout (labelled "Single Coil Type II") at: http://www.geocities.com/BourbonStreet/Quarter/9440/pickup.htm  
 
Also, Steven Kersting (owner of the first page) shows a pretty good pictorial on building a P-90-like Strat pickup he calls the "S-90."
 
11/15/2004 9:16 AM
Kevin
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Jason Lollar has this on his website.  
 
http://www.lollarguitars.com/BobbinDesigns/p90_bobbin.jpg
 
11/15/2004 1:28 PM
John Culp
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Thanks for that link, Kevin!  
 
Am I understanding that correctly, that the bobbin spacer of Plexiglass is the core of the coil, the only magnetic element inside it being the pole piece screws? No steel slug?
 
11/15/2004 1:30 PM
jason lollar
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A gibson P-90 would be a one piece cast plastic bobbin, essentially the same as the drawing, only the screws are inside the coil.
 
11/17/2004 7:55 AM
Mark Hammer
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WARNING! THE FOLLOWING POSTING CONTAINS SCENES OF PICKUP SACRILEGE, VIOLENCE, COARSE LANGUAGE, EXPLICIT SEXUALITY, AND NUDITY. READER DISCRETION ADVISED.  
 
An "orthodox" P-90 utilizes magnetically conductive (but nonmagnetized) polepieces, with two bar magnets oriented nose to nose at the base of the polepieces to produce a sort of inverted T-shaped arrangement. The top of the polepieces is one pole, and the outward edges of the bar magnets is the opposite pole. Although it would likely be not particularly cost effective to do so, the same thing could be achieved if one fabricated a single-piece alnico T-shaped channel bar, cut slugs off the bar the same length as the pickup, and then magnetized the slugs so that the top of the T was one polarity and the stem the other. The stem of the T slides into the slot in the middle of the coil and away you go. (Okay, that covers off both the sacrilege and coarse language parts - I can hear you all muttering now)  
 
While not 100% of the formula, it is the fact of the different sensing area that gives the P-90 some of its unique character. The sensing area is no longer from the top around the sides to the bottom of the coil, as in a Strat, but rather between the top and sides. The resonances of the coil, coupled with the different sensing area allow it to pick up more of the mids and provide a meatier, toothier sound.  
 
There are, of course, many different ways to produce that sensing area difference. The classic is what you see in the P-90. The hypothetical T-channel "all magnet" slug is another. A third is a standard bar magnet on its side with passive conduction of the opposite pole out to the sides by a bottom plate of some sort. There are likely others I'm not clever enough to think of. I suspect they would all yield audible differences, but would all share that broad category of sound that is characteristic of single-coil sideways-sensing pickups.  
 
Years ago, I had a sweet Epiphone semi that came with one of those New Yorker pickups that have real ivory binding and look like tiny humbuckers with the screw polepieces ridiculously close to the edge. In fact it was NOT a humbucker, but was a single-coil PU with a flatish coil. The magnetic field was arranged such that the coil had one pole peaking out the top (but under the pickup cover), and the bottom plate *conducted* the opposite-pole out to the side to the bracket where the adjustable screws were threaded in (Hmmm, maybe I'm confusing what I found when I pulled a cheap Harmony pickup apart, but whether it was a bar magnet connecting the poles at the side and centre-coil or a conductive plate, it was still unidirectional rather than out both sides like a P-90). Rather than being a strictly top-to-bottom sensing area as in a Jazzmaster or Strat, OR a side-to-side sensing area as in a P-90, it was a lopsided-sideways sensing area. As a single coil, it had an extremely beefy toothy sound, sharing much more in common with a P-90 or humbucker than with either a Jazzmaster or Strat.  
 
The moral of the story? Ponder ways to direct the "other" pole out to the side. The stock P-90 represents one of the ways, but there are others that may be equally productive in terms of bringing you in the direction of a tone with a pleasing substance to it. Sideways-sensing has plenty to recommend it and much to still explore.  
 
So where was the nudity and explicit sexuality? Well, you can't see what I'm wearing or what I'm doing right now. Just as well. For many of us, as well, I don't think there CAN be anything more explicitly sexual than a great-sounding instrument!
 
11/17/2004 12:11 PM
jason lollar
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"Although it would likely be not particularly cost effective to do so, the same thing could be achieved if one fabricated a single-piece alnico T-shaped channel bar, cut slugs off the bar the same length as the pickup, and then magnetized the slugs so that the top of the T was one polarity and the stem the other. The stem of the T slides into the slot in the middle of the coil and away you go."  
 
Well even though it may have a similar function if it could be polarised in that way the result would be quite different- first the loading of the coil would be far different and the gauss level at the end of the pole would be different than a magnet contacting a screw.
 
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