How to detect polarity of pickups for cancelling hum?

Death Incarnate

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I am a big fan of P-90s, and have equipped a few guitars with humbucker slots with Gibson P-94s. The first I time I did it in my 1998 ES-335 LE, I put a P-94R in the neck and P-94T in the bridge and it was great. The R&T are wound to opposite polarity, so when the pickup switch is in the middle position, there is zero hum, just like how a humbucker works cancelling the buzzing.

But I've bought four P-94s, a mix of new and used, two were labeled P-94R and two P-94T. However, they are not working that way in two other guitars - an Epiphone Black Royale Dot and Gibson ES-335 Dot. There is still buzzing in the middle position. I suspect some are not wound opposite, despite which pickup name I was sold and what the white sticker indicates on the back. Maybe some of them are not even actually a Gibson P-94, despite looking identical. Wiring stuff with 335s is such a nightmare, so I would prefer to save myself some stress figuring this out, rather than attempting through trial and error to figure out which pickups are which, and figure out why this is not working.

Some questions:

1) How can I detect the polarity of the pickup? Googling this says I can use a compass over the pickup, to detect if it's pulling north or south. Does this really work? Can someone elaborate - as long as the neck is pulling the opposite of the bridge, it should always cancel in middle switch position?

2) Is there anything specific about how the pickups wire to the pickup switch that has to be done for this to work? Anything specific about how the switch operates that is required for this work?
 

cooljuk

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You're not going to be able to do this without some trial an error, since you don't know for sure what pickups you have.

There are TWO properties that have to come into (or out of) alignment for you to get the pickups both in phase and hum canceling in the middle position.

They need to be electrically out of phase and magnetically reverse polarity. A compass will only tell you magnetic polarity, which won't get you there, alone.

If they are presently working in a guitar with both pickups in phase with each other in the middle position, but not hum canceling, you need to reverse both the electrical polarity and the magnetic polarity on either pickup. That will keep them in phase, but cancel the hum.

To reverse the electrical polarity, you need to get inside the pickup cover and do a mod to the small wires coming off the coil. Unless your pickups have four conductor wiring, this can't be done any other non-invasive way.
 

cooljuk

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You can test this outside of the guitar, though. You can hook up both pickup lead grounds to each other and the ground of a guitar cable going into an amp. Also hook up both pickup lead hots to each other and the hot of the same guitar cable into an amp. The way the pickups respond in that situation on your bench (humming or not, in phase with each other or not) is the same way they will respond in your guitar in the middle position.
 

Rocco Crocco

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cooljuk

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I bought this magnet on a stick thingy from Philadelphia Luthier. It works but it has a cheap plastic handle. Link below.
Just like a compass, this only tells magnetic polarity and says nothing about the electrical polarity. Both must be correct for the desired effect.

I think you can flip either pickup 180 degrees in a guitar to get hum cancelling. No wiring changes needed. If not someone will correct me.
Nah, that won't do anything at all.
 

NotScott

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If you want to determine electric polarity before you assemble a set in a guitar, this is quick, easy and accurate:


Used with a compass or a bar magnet, you can quickly verify electric and magnetic polarity.
 

cooljuk

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Some meters will lie to you with that method. It'll appear to work but, because digital meters can be slow to calculate/display, the latency of the meter makes it appear like the voltage is going in the other direction.

An analog meter will show voltage pushing the needle forward or in reverse but most don't have those around anymore.
 

NotScott

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Some meters will lie to you with that method. It'll appear to work but, because digital meters can be slow to calculate/display, the latency of the meter makes it appear like the voltage is going in the other direction.

An analog meter will show voltage pushing the needle forward or in reverse but most don't have those around anymore.
True, some meters are dogs, but I have used this method for a while with everything from Flukes to Radio Shacks to Amazon specials and it has always worked for me.
 


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