Photo 1 — Courtesy of goeldo.de
Recently, I showed you how to reduce unwanted noise in single-coil pickups without ruining their signature clarity and sparkle. (If you missed this, check out “How to Shield Single-Coil Pickups.")
The next logical step is to explore ways to include pickup covers in our single-coil shielding scheme. Here's why: After shielding a single-coil pickup with copper tape, its top—the area with exposed magnets—remains unprotected. This provides an excellent portal for hum and noise to creep in.
On a standard Strat or P-90 pickup, the pickup cover protects the windings from physical damage. Wouldn't it be great if we could expand the cover's utility to further enhance the shielding, even if the cover is plastic? We can, but there's a trick to it. If you're not careful, all that single-coil chime can go down the toilet. I'll show you how to avoid this by following the same principles we used for shielding the pickups themselves.
To summarize what we discovered previously: If you wrap a pickup with copper tape and the two ends make contact with each other, the loss of high-end is unavoidable—just as it is with a completely closed metal pickup cover found on vintage-style humbuckers and Telecaster neck pickups. This is caused by eddy currents in the metal, which act like a “shading coil" in a transformer. A gap in the copper tape breaks the eddy currents, reducing high-end loss to a minimum.
Putting a metal cover on a pickup adds a good dose of shielding, as long as the cover is connected to ground. Typical examples are the aforementioned PAF humbuckers and Tele neck pickups, but their covers dampen high end, which explains why so many players remove them. When you remove the cover, the difference in tone is stunning. Wouldn't it be great if you could get the benefit of shielding without compromising the treble frequencies?
Shielding background. The degree of treble loss is primarily determined by the type of metal used to make the cover and how thick it is. Michael Pantleon, the pickup expert from the LeoSounds, describes it this way:
plastic or wood.
The loss of high end through eddy currents is unavoidable when the pickup cover is made from an electrically conductive material. The following rules apply to this behavior:
· The higher the frequency, the greater the loss.
· The higher the ohmic resistance of the cover material, the less high end will be lost.
· The thinner the cover material, the less high end will be lost.
The metal cover's core material has a crucial effect on the pickup's sound. If you want to minimize high-end loss from a metal pickup cover, a really thin shell made from German silver (an alloy of copper, nickel, and zinc that's also known as nickel silver) is the way to go. Conversely, a really thick and heavy brass cover creates maximum high-end loss.
A metal cover's plating has a minimal effect because it is very, very thin. In terms of high-end loss, nickel plating creates the least, followed by chrome and gold plating, which has the greatest treble-reducing effect.
As for shielding against external electrical fields, brass offers slightly better shielding compared to German silver. It's important to note that all pickup cover materials, such as German silver, brass, or aluminum, are not ferromagnetic. A ferromagnetic cover would eat up all magnetic field lines and render the pickup useless.
So, one possible shielding strategy for single-coils is to find a metal cover that has properties that enhance shielding without losing too much high-end chime. Another option is to shield a plastic cover, which we'll learn how to do in a moment. But first, let's explore the commercial products that are available for Fender-style single-coils.
Photo 2 — Courtesy of allparts.com
Open-frame covers. Essentially, these are metal rings with an open top (Photo 1). With these covers, the top of a pickup is still exposed, so hum and noise can creep in, but they are a good substitute for wrapping copper tape around a pickup, and they look great, too. Open-frame covers are available for different pickup types.
But because this design is a closed ring without a gap, it will kill some high end. As we noted earlier, the type and thickness of the material used for the cover will also influence the sonic outcome. Such open frame covers are also available for Telecaster neck pickups, and the improvement in tone is impressive. On the other hand, you lose some shielding because the top of the pickup and the magnets are no longer covered with metal. So you have to decide what is more important to you—tone or shielding. And if you install a metal open-frame cover, remember it must be connected to ground. Otherwise it's pretty useless as far as shielding and will eat up your high end without providing any benefit.
Metallized plastic covers. These plastic covers for Strat (Photo 2) and P-90 pickups are available in gold and chrome, and they add a classy touch to a guitar. But be aware: Such covers contain a lot of metal flakes (some are even conductive), so if you surround a single-coil pickup with a metallized plastic cover, it will behave like a closed metal cover and reduce your high end. But metallized covers can add extra shielding to your pickups, as long as you connect the covers to ground. Of course, that's tricky because they're plastic and you can't solder a ground wire directly to them. The workaround is to use conductive glue to attach a thin wire to each cover, and then connect those wires to ground.
Shielding a standard plastic cover. Okay, project time! Let's learn how to shield a standard plastic cover without losing too much high end. To illustrate the procedure, I'll use a standard Stratocaster pickup cover, but this will work for any pickup cover made from plastic or wood. You won't need much in the way of tools and materials—just some conductive, self-adhesive copper tape, some alcohol or naphtha to clean the area where you'll apply the tape, a pair of scissors and a scalpel, a fine-tip Sharpie, a soldering iron, and some thin insulated wire. Study (Photo 3) for a moment, and then I'll explain how we get there.
Photo 3 — Courtesy of singlecoil.com
Step 1. Measure the depth inside the pickup cover and then determine how long a strip of copper tape you'll need to fully encircle the cover's interior sides. Using these measurements, cut a strip of copper tape, trim it to size, and then compare it to the cover. If the tape is too tall, no problem—you can easily trim away any excess with a scalpel in the next step. Finally, cut off a little piece from the long side of the tape so the ends won't touch after you've wrapped it around the interior.
Step 2. Adhere the copper tape to the cover's interior sides, placing the small gap at one of the round corners. Trim off any excess with a scalpel, so no copper tape protrudes outside the pickup cover.
Step 3. Place the top of the pickup cover against a fresh piece of copper tape and cut out its outline. As you do this, press the top of the pickup cover firmly against the tape so you can clearly see the imprint of the six holes. Now cut the copper tape lengthwise, exactly down the middle, to create two pieces. With a fine black Sharpie, draw a line around the imprint of the holes, and then snip away this part with a small pair of scissors.
Step 4. Stick the two top pieces inside the cover and choose a solder point where the top and side pieces contact each other. Solder a thin piece of insulated wire to this spot. The other end of this wire has to be connected to ground, i.e. the back of a pot. Be very careful while soldering: You'll need to work quickly so you don't overheat the cover and melt the plastic.
As an alternative to copper tape, you could use conductive silver paint to shield the interior of the pickup cover. When applying the paint, follow the same pattern as described for the copper foil and remember to maintain the small gap described in Step 1. Conductive silver glue works great to connect the painted interior to one end of the ground wire. From my experience, copper foil works much better in terms of shielding, but it's easier and faster to apply the conductive paint, so that's a choice you'll have to make.
That's it. In our next installment on shielding, we'll deal with the pickguard, followed by the electronics cavity, which will be the fourth and final exploration of this subject. Next month, however, I'll show you two completely new pickup-selector switches that recently appeared on the guitar horizon. This is something many players have been waiting for, so it's really exciting. Until then ... keep on shielding!