For quite a long time, I have been wondering about a phenomena called “eddy current losses” in guitar pickups. Essentially, these are signal losses due to various electromagnetic aspects of the pickup’s construction, that cause the tone to become dulled to some degree. All pickups have them, and some depend on a calculated amount of losses to produce a certain tonal balance. However, because the high audio frequencies are conducive to a sensation of “clarity” or “brilliance” in the sound, it is generally good to reduce the losses as much as possible.
While analyzing pickups and examining the analyses of other testers, I began to realize that the metallic covers that contain the coils and internal parts of a pickup, are a prime source of loss. This was actually known to the early designers of the 1950’s era. They responded by finding metals that have low losses, and used those as the base material for pickup covers. These would then be electroplated to any desired appearance.
J.R. Butts, a designer for the Gretch guitar company, chose a different way. He considered the electromagnetic problem more carefully, and designed a metal cover shape that was almost completely immune to the losses – the “Filtertron”. Subsequently, the Fender guitar company adopted the design for a specialty guitar – the “Cabronita”.
After 1960, nobody thought much of the whole thing. High quality pickups always used an alloy called nickel-silver, while the Filtertrons and some other covers remained the more inexpensive brass. But when brass is used in a non-Filtertron design, the sound is usually very dull due to the eddy current losses.
I wondered, why does the J.R. Butts design work so well? The patent mentions it, but offers little explanation. So I began some experiments to determine the exact nature of the eddy currents in a guitar pickup cover. These were extremely revealing. Soon, I realized that the Butts design barely scratches the surface of the techniques that could be leveraged to improve a pickup cover.
I designed and built several prototype alternative designs made from brass to test my theories. These were wildly successful. However, as I considered what I would do with my invention, I realized that I lack the funds and resources to obtain patents, trademarks, set up inventory, place manufacturing orders and such things that are necessary to make and sell a product.
So after almost a year of development, I feel that the best course of action is to simply release the information into the public domain. I hope that if it has some small success as a product, that I can at least boast that it was my idea. After all, it probably won’t be the last one coming from me.
The technical article is long, so I should give you a summary. The idea is that by cutting small slots in strategic locations on the cover, the tone-sucking eddy currents can be mostly eliminated. This has two applications. One is that cheap brass covers can be used where a nickel-silver one would normally be used. That is a cost advantage. Another is that when a nickel-silver cover is slotted, the losses are so small as to be both non-measurable and inaudible. This means that a protective cover can be added to pickups that have previously shunned covers for reasons of tonal purity (this habit began with the heavy metal players of the late 1970’s).
Here is the full story: pickup_cover_geometry