dummy coil tele testing

[Rob DiStefano]

On the newer designs the bottom coil is a dummy. You are using a dummy. What's the difference? None.

The dummy coil is not picking up the strings, so there is no frequencies being killed off. It's only picking up hum.

yeah, i knew that, of course

Not if the dummy coil is shallow enough. You think routing a humbucker cavity is more work than routing an existing cavity deeper??

what? i have no concern over routing cavities in guitar bodies, but i don't wanna change the dimensions of an existing pup cavity


Why even bother with the bobbins then? You can wind a coil around anything.

yes, i know that - yer missing the point of this, my first dummy coil exercise - i had some bobbins gathering dust, i used one that was pre-wound for testing, can you not understand that?

I'm sorry but a lot of what you are saying seems very arbitrary.

hello david? that's the whole point of what i'm doing, testing stuff out that to see what happens, i've never messed with dummy coils, arbitrary and all, isn't that the point of personal testing and trialing? why the attitude, dude? didn't you ever test crap out? yeesh, i'm looking for constructive help, maybe some encouragement and not attitudes. have fun, life's too short, i'm outta here.

....

[David Schwab]

hello david? that's the whole point of what i'm doing, testing stuff out that to see what happens

Yeah, I test stuff out. But you have to follow some kind of reasoning. In some ways you are reinventing the wheel. You aren't the only person here who has experimented with dummy coils. So what I am giving you is constructive help. But you dismissed most of it. Mike has dummy coils in his guitar. Rick Turner used some of the first dummy coils back with Alembic. Everyone is giving you advice.

You are doing some things the hard way. Why attach two humbucker bobbins together when you can get anything... a cardboard tube, a piece of wood.. and wind a coil on it. The coil can be any shape. Round is easy.

Then there was the idea that routing a new cavity is easier than making one deeper, even though the existing one will act as a template for a pattern bit.

Then you say stuff like: "but i don't wanna change the dimensions of an existing pup cavity" Why? Will it alter something about the pickup? I think it's beter than chopping a new hole in the body. See? Arbitrary. You know the dummy isn't picking up the strings, but you said moving is closer will stop it from killing off frequencies. No, it wont. It's the change in inductance you are hearing.

[Mike Sulzer]

Mike is absolutely correct...to a point, and that is that if your idea of tone includes the capacitive loading of internal wiring and then a guitar cable, you'd have to monkey with some caps to make an active hum canceling system sound like a single coil pickup with no hum. Guitarists are incredibly used to the tone that comes from a bunch of things being "wrong" with high impedance guitar and bass pickups. You've even got raging arguments here over perceived tone differences from simply changing from one supplier of magnet or a formula of pole piece alloy, to say nothing of active vs. passive differences or of putting a coil in series with another, no matter the impedance of the different coils.

I must say, I like that deal with the P-90 hum canceling system. You could go RWRP with one P-90 and then switch the hum canceling coil in and out depending on whether you were using one pickup or two. P-90s just sound so good...

No monkeying necessary if the active preamp amplifies the dummy only. The ground lead of the pickup is removed and connected to the very low impedance output of the dummy preamp. The commercial system for doing this was described in a thread here recently, and I have my own simpler one. One FET is all it takes.

[Rick Turner]

Understood, Mike. The hum canceler buffer/preamp is driving the pickup, and the pickup impedance is basically unchanged. Missed the discussion on that somehow. Makes sense. I suppose you could switch gain as you switched pickups as well and not have to use an RWRP configuration.

[Sweetfinger]

My experience has been that the closer the dummy coil is in construction to the the pickups, the better the hum cancellation. I tend to like putting the dummy coil somewhere else than next to or under the pickup as well. When you place a dummy coil near the pickup it will invariably pick up a little string signal due to the stray magnetic fields surrounding the pickup. I have always found that a separate dummy coil retains more of the SC character than a stacked pickup which is why I don't tend to "stack" dummy coils under a pickup unless space or logistics dictate. I also try and keep my use of core material to a minimum in the dummy.

[Mike Sulzer]

Understood, Mike. The hum canceler buffer/preamp is driving the pickup, and the pickup impedance is basically unchanged. Missed the discussion on that somehow. Makes sense. I suppose you could switch gain as you switched pickups as well and not have to use an RWRP configuration.

Right, no need for RWRP. You feed all the pickups from the buffer. I use a separate low resistance pot on each for individual adjustment. (I have only done this with two, but it ought to work with three.) Then you cannot use RWRP because the hum would add into the RWRP pickup, not subtract.

[Mike Sulzer]

My experience has been that the closer the dummy coil is in construction to the the pickups, the better the hum cancellation. I tend to like putting the dummy coil somewhere else than next to or under the pickup as well. When you place a dummy coil near the pickup it will invariably pick up a little string signal due to the stray magnetic fields surrounding the pickup. I have always found that a separate dummy coil retains more of the SC character than a stacked pickup which is why I don't tend to "stack" dummy coils under a pickup unless space or logistics dictate. I also try and keep my use of core material to a minimum in the dummy.

Your last sentence contradicts your first. The core affects the sensitivity to magnetic fields. You really do want the pickup and the dummy to have the same sensitivity; so if you are going to make then the same, they need the same core..

[Mark Hammer]

Think of the dummy and the sensing pickup as tuning in a radio station: Radio WHUM, 60 on your dial. The goal is that they both pick up the same signal with the same strength, so that when wired antiphase, there is near full cancellation. Having a near-identical replica of the sensing pickup right beside the sensing pickup is one way to do that, but it is only one way, and not the only way, or necessarily the best way.

[greenfingers]

greenfingers, wouldn't all that shielding paint in the control cavity reduce the effectiveness of the Ilitch/Suhr coil? You'd just be picking up the noise signal from the rear of the guitar?

That's not my photo, it's one from the link i put at the bottom, I don't think it's shielding paint, looks a bit like foam, maybe the coil comes mounted on it?

[Dave Kerr]

gf, shielding paint or not, Rick pointed out that it shouldn't matter an awful lot.

[Rick Turner]

The core of the dummy coil CAN affect the relative response to 60 Hz harmonics by changing the inductance. These days you're not just dealing with straight 60 Hz; you've got the multiples, and then you've got all the weirdness from computers and other digital hash in the environment.

The Alembic dummy coils are/were wound on Plexiglas while the pickups are/were wound directly on ceramic 5 magnets. They work(ed) pretty well.

BTW, my 1969 "Pretzel Guitar" has what I think are the earliest dummy coil hum-cancelers that we did; that guitar is really the last pre-Alembic, Inc. instrument I made, but was done with preamps that Ron Wickersham designed also in 1969. The dummy coils are passive in series with each pickup, and the pickups are my original trapezoidal ones hand wound...literally. That guitar will be on exhibit next year at New York's Museum of Art and Design in a show, "Crafting Modernism, American Arts and Crafts from 1945 to 1970".

[David Schwab]

The dummy coils are passive in series with each pickup, and the pickups are my original trapezoidal ones hand wound...literally.

I remember reading you were winding a coil and your wife came in and said something and you lost your count and had to start over?

I love the trapezoidal pickups.

[Rick Turner]

Yep, that's it and that's what happened!

Note the "roach clip art" tailpiece...

The pickup shielding is kind of Faraday in that it's a spiral wrap of 1/4" copper foil tape with gaps, and it's only connected to ground at one end. It was an early idea to try to reduce eddy current loss, but it didn't seem to make much difference... Sometimes you've just got to try these things. The pickups were cast in polyester resin, and everything still works fine.

[David Schwab]

Yeah, I was using fine brass screen as shielding for a while, but I can't say I noticed a difference.

Those are cool instruments.

[Sweetfinger]

Your last sentence contradicts your first. The core affects the sensitivity to magnetic fields. You really do want the pickup and the dummy to have the same sensitivity; so if you are going to make then the same, they need the same core..

There's not a contradiction- The best hum cancellation occurs with a core, but then there are unwanted effects from increased inductance. It's a trade off. I find somewhere in the middle that I can live with and call it good.