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Old 12-14-2008, 09:59 AM   #138
J Kennedy
Human being with feelings
 
Join Date: Sep 2006
Location: ocean mists
Posts: 858
Default single coil pitch/volume distortion

This is a problem that has been around a long time and was I think first reported in Fender single coils with the introduction of Alnico magnets. As said, the style imposes how much of a problem it is, specifically if you play a lot of barr chords high on the neck. The guitar will never be in tune unless the pickup is backed down away from the strings.

We modified a Gibson SG with stock P 90ís, replacing the coils with Dirtyfingers. The guitarist trades out guitars between his Gibsons and a 2008 American strat. He was having trouble with the volume difference between the guitars and the Dirtyfingers made the gap wider. We did the neodymium additions to the strat and raised the pickups. Center tone was sacrificed for a 1+3, 1+2+3 blend. The strat was up there in lights with gain equal to the Gibson with incredible tone. Sounded good at my place anyway.

He got them home and cranked them up thru a Marshal stack, started playing the high barr chords (which I usually donít). Got the call that something was seriously screwed up, everything out of tune and a major tremolo effect going on.

Some trivia on why this happens and reason to see if your guitar is doing this even stock off the shelf.

Double coils arenít as much a problem because the magnets are usually bar types on the bottom of the pickup and a weak flux going thru the screws and poles. Single coils are often wound around the magnet itself and present a strong field coming direct at the string. Field of magnets drop of at the 1/x2 ratio or whatever so that smaller distances mean greater field difference the closer you are to the pole.

Pick a string and we get the normal fundamental and harmonics but thereís also a distorted oval motion going on like a jump rope. The jump rope swings close to the pole and there is a marked increase in volume, several cycles per second giving a tremolo. On the downward pass, a strong magnet pulls the string down and raises the pitch throwing everything out of tune. Sounds bad.

The effect is heard higher up because the strings are pressed closer to the magnet, but mainly because the center point of the string with the biggest displacement is shifted over the pickup. The stress also sets up a node for weird harmonics that wouldnít usually be there. Mainly the low E and A are affected, 10th or so fret up and worse with lighter gage strings.

Fix is to lower the bass side of the pickup until the effect disappears or donít play high up barr chords. Note that adjusting out the pitch distortion has to be done with all the pickups. Neck pickup is worse than a middle pickup. A pickup close to the bridge has to be really strong to have much effect, but you can be playing the bridge pickup, getting the warp and the problem coming from a neck pickup thatís not even in the circuit.

About the only other thing I have of worth about any of this is a simple mod on a strat that makes a big difference in being able to cut through the computer environment. Solder in two .005 mfd capacitors in series across the two non-grounded terminals of the volume control. This gives you the equivalent of a Tele filter at about half the value, passing higher frequencies. Best effect for me is in the volume range between 7 and 10 where you use the volume control to adjust the bite. Gives a lot of fine control over what goes into compression and distortion effects.

Best,
John
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