P.A.F./Tele/Strat Split Coil Mod for SH-4 JB

Posted: March 30, 2015 in DIY, Guitar Wiring Mods, Ideas To Be Developed, Modification
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Vintage humbuckers and those which emulate P.A.F.-style humbuckers have a distinct dip in the midrange, due in part to the unbalanced DC resistance of the coils and the number of winds on each bobbin.  Many modern humbuckers are over-wound, but still have an unbalanced pair of coils to produce a “P.A.F.-on-steroids sound”.  Many other modern humbuckers are fairly well matched, giving the pickup a spike in the midrange.  The Seymour Duncan SH-4 JB is one such pickup.  At a total DC resistance of 16.4k, it’s more than twice as powerful as a P.A.F..  My idea was to create a permanent offset and have a spin-a-split that doesn’t reduce to zero.

Not a lot of care was always exercised when winding vintage pickups.  That’s why there is such a variance between humbuckers from the same factory, even from the same production batch.  Offsets are at least 3% to 5%.  A couple of production “vintage” pickups are offset much higher; 10-15% unbalanced and, rarely, up to around 20% at the most.

In general when doing this mod with a spin-a-split, the higher the offset, the less midrange and more treble there is.  The bass will have more definition up to a point before it begins to be removed entirely. The effects are due to a combination of phase cancellation and that the second coil becomes too weak to transfer the vibrations of the strings and the actual size of the transfer field reduces as the pot is turned off.

The SH-4 JB has fairly evenly matched coils and a pronounced mid-hump, which makes it perfect for customizing into a hot-rodded P.A.F with a modified “Spin-A-Split” to get single coil tones, but not sacrifice all the hum canceling properties of the second coil.  I like the regular S.A.S., but found myself always turning the knob until the humbucker was working a little to provide a better tone.

What I did was this: I used a linear pot and a resistor to tap the second coil, but prevent it from completely shunting to ground at the lowest setting.  This allows the pickup to be a pseudo-single coil sound, a full humbucking sound, and anywhere in between.  For my purposes, I used a 25k linear pot and a 10k resistor.  To do the same modification, but keep the humbucker close to it’s original sound, a 250k or 500k pot could be substituted, because it will not reduce overall resistance as much as the lower value 25k pot.

SASR

Because the resistor and pot are placed between the coils in parallel, only coil #2 is affected and all measurements are assuming an 8.2k DC resistance from that coil.  Due to tolerances of the parts, a small variance from the calculated values is perfectly fine.  I’m also going to round a bit, since the real world application will not be perfectly matched to the assumed values.

Resistances in parallel divide to a point lower than the lowest resistance.  The 8.2k coil “sees” 35k in parallel and it reduces its own value to about 6.6k (14.8k output from both coils).  At it’s lowest setting, the 8.2k coil will “see” only the 10k resistor and reduces to 4.5k (12.7k total output).  The linear pot will give a smooth taper for finding sweet spots in between either output.

For the more mathematically minded, 6.6k is a 20% difference from the expected 8.2k of coil #1.  This is more than most P.A.F.s, but still works, since it compensates for the more compressed nature of the SH-4.

With the pot all the way off, the psuedo-single coil sound is very bright and the bass is cropped.   It spanks like a Tele and does so with added bass as the pot is moved to about 2 or 3 (3 is a sweet spot on mine).  When moved a little higher, it takes on a more robust sound and is more like a Strat, but uses two coils (sweet spot around 5).  Moving higher still, it’s begins to sound like a hot-rodded P.A.F. from 8-10 (10 is SWEET!)  Its output is still modern (hot), but the emulated tones are very apparent.  The full-on sound has a tight bass, trimmed mids, and is bright without being brittle.  The pick attack is clear and it cuts through with heavy distortion.  I love it.

All sounds are hum canceling, but to a lesser extent.  I didn’t pick up any 60-cycle hum, but the t.v. and a light provided some noise when I turned directly toward them, regardless of the pot setting.

The values can be changed to fit a person’s needs or style.  A slightly larger resistor will prevent the bass cut.  A large value pot will decrease the unbalance at full on.  The value of the pot and resistor can both be changed to reach a specific balance between the coils.

Just remember, the resistor and pot are in series to each other, but parallel to the pickup.  The general idea is more useful than following my adoption of it.

[Edited 8/20/15: Corrections to concept and honest mistakes]

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Comments
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