DIY electric guitar projects: Dummy coil for Danelectro

Hi,

this is yet another Danelectro upgrade that I did recently. This guitar was always kind of noisy and hissy, but I did not really mind, but then we were playing at a gig where the lighting and other electronics created a lot of bad stuff that got picked up by the lipstick pickups, fortunately I was standing far away from my amp otherwise it would be a disaster, not only it would hum but also squeal. After that I was recording some guitar parts and again, it was in a place that has a lot of fluorescent lights, computers, fans etc. When the guitar was positioned so that the noise was the worst, it was half as loud as the actual playing! That was really insane. So I did some research and I found a pretty nice solution produced by Suhr company for Stratocaster guitars and similar thing made by Ernie Ball that is used in their guitars - a dummy coil.

I won't go into the actual guitar pickup functionality, it is explained in many other places around the internet. But basically, every pickup picks up a noise/hum. If you want to cancel the hum, then you need to pick it up again, but out of phase, so it cancels itself out, like if the noise was a sine wave, one coil would pick up a top peak and another coil the bottom peak, which would leave zero signal. This is how the humbucker pickup works, you have (usually) two more or less identical single coil pickups that are wound in opposite direction (magnet polarity is also reversed so the actual audio signal from strings does not cancel out also). If you for example remove the pole pieces from one coil of a humbucker, you would get a "single" coil that is cancelling out hum. But since the signal goes through both of these coils, the resistance and inductance of the other coil also affects the sound. So what I'm trying to say is, that you need to find some balance between hum cancelling effect and a tone changing effect, ideally, you want 100% of the first and 0% of the second effect, but that is just impossible. But you can get pretty close.

Ok, so I told the theory anyway, might as well continue. There is a patent here, that explains one very basic physics principle regarding coils, particularly guitar coils and them picking up noise.

http://www.google.com/patents/US7259318

It says, that you can create a coil, with much less resistance and inductance, thus a coil that would not affect the tone (it does, but you can't tell the difference). All you need to change is the coil area, the area inside the coil. This area for a guitar pickup could be, let's say 10 cm2. The pickup has 5000 turns of wire and for example 5k ohms. If we were to create a coil, big coil that would fit inside the guitar, under the pickguard or backplate (the Suhr way) with area let's say 100 cm2, then the dummy coil would need only 500 turns. And here comes the best part - the wire gauge doesn't matter, so you can wire it with much thicker wire than the pickup, so you end up with a dummy coil that has a resistance like 50 ohms and also negligible inductance, so your tone gets changed very, very little.

Back to the Danelectro. Calculations: bridge pickup is 4,02k ohms, I estimated 43 AWG wire gauge, 9,75 cm2 area of the coil, 16 cm perimeter of one turn. In some tables you can find electrical resistance of copper wire, for 43 AWG and my pickup, it results in 512 m of wire. That divided by 16 cm leaves 3200 turns. Multiplied with the area it leaves 31200 cm2*turns - this is the hum picking value of the pickup. Save this value for later. (All of this is questimating, because the effectiveness of a coil comes into effect too, pickup coil is not very effective because it is far from round, these calculations are done for two coils with the same shape).

Now where to put the dummy coil, how should it look like? How big? This was actually, for the Danelectro very challenging, there is no backplate to use the Suhr design, I can't just remove a magnet from some other lipstick pickup because there is no plastic bobbin inside of these, in order to remove the magnet, you destroy the coil. Danelectro is hollow, but you can't really reach inside that well.

Article about installing dummy coil inside a Stratocaster (there is also another article about Telecaster on that site) https://sites.google.com/site/stringsandfrets/Home/noise-reduction-for-sc-pickups

I decided to put the pickup inside, using a plexiglass bobbin that I made for this. After careful measuring, it was clear that I could fit a round coil with 5 cm in diameter max. Back to the calculations, I have the 31200 cm2*turns, this divided by area of a circle that has 5 cm in diameter (20,43 cm2 area) leaves aprox. 1530 turns. That's a lot, but this is the biggest coil I can fit inside, so decided to go for it.

I had a small spool of 0,22 mm (32 AWG) enameled wire that I used, after some 1500 turns it was clear that the spool could give me around a hundred more turns, so I used it all. I then used a very simple setup to test the hum cancelling qualities using a breadboard, PC soundcard oscilloscope and a guitar amp. First I measured (this oscilloscope is very crappy but just for comparing hum levels it is fine) the response of a bridge pickup, amp was set to very high gain distortion channel so the hum is amplified a lot and therefore visible in the scope. I got around 120mV p-p. Then with the bridge and neck pickup in series (which formes a humbucker) it gave me 55mV p-p. So I was aiming for this value with the dummy coil and a bridge pickup in series. First, it was around 65mV p-p, that's not bad. Then I removed 50 turns from the dummy coil and measured again, now it was 55-60mV p-p, another 50 down, now I had that 1500 turns which was calculated, it gave me 50mV p-p, which is even better hum cancelling effect than the humbucker setup! This is probably due to the fact that Danelectro pickups are not identical like the two single coils in a humbucker. I was customizing it for the bridge because I use it for gainy sounds and the neck for clean. Resistance of the dummy coil is 140 ohms.

Although the mains cycle hum got cancelled completely, I must have some weird source of EM interference at home because from time to time there is some weird noise coming from the single coil (tested with my Les Paul tapped humbucker to test if it was really some interference and not the guitar). This noise is not present in our rehearsal room. Interesting fact remains though, that this noise gets cancelled with the bridge and neck pickups in series, even though the mains hum is cancelled less. This is probably because of the different shape, wire, and other things of the dummy coil.

Sound is awesome, everything works fine. Position of the dummy coil in the circuitry required no difficult changes, as you can see from the schematic, when you select the bridge pickup, the dummy coil is in series, when you select the middle position (humbucker - neck and pickup) one end of it is not connected and when selecting the neck pickup, the current flow reverses so the dummy coil changes polarity and works for the neck pickup too, which is awesome. The switch in the schematic is ON-OFF-ON type, sorry I could not find the correct schematic symbol for it.

Also I changed the potentiometers, because I just don't think that the volume and tone for each pickup is practical. The first schematic is the original, the second is the upgraded. Potentiometers in the second schematic are done in the 50's style, so that I won't loose highs when rolling down volume. And also the tone control became actually usable, it now has much better control and range, I might learn to use it after all!

Pictures: