Those who are familiar with Sir Brian May's iconic Red Special guitar will know that is fitted with three Burns Trisonic pickups, each with an on/off switch and a phase switch, The phase switch effectively reverses the pickup wiring. When the phase switch is set the same on two pickups, they're in phase; when it's different, they're out of phase. Various combinations of pickups — turned on/off and in/out of phase — allow Sir Brian to create his extensive range of sounds.
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| Kent Armstrong Tri-Sonics |
Sir Brian famously purchased his Tri-Sonic pickups from Burns of London for a total of nine guineas (£9 9s) and then modified them to produce the sound he wanted.
An early issue he encountered was microphonic feedback at high gain levels, resulting in an unpleasant high-pitched whistling noise. This was caused by microphonic vibrations in the pickups. He discovered that pressing his hand on the pickup causing the feedback stopped the vibrations and resolved the issue. To permanently fix this, he inserted tape between the pickup casings and magnets and filled the air spaces in the bridge and neck pickups with Araldite (a type of epoxy) to dampen the vibrations. This successfully eliminated the bad feedback while allowing the pickups to produce musical "good" feedback without whistling.
Going back to pickup phasing: not only does pickups being out of phase produce different tones, it effectively flips the pickup polarity (making the pickup combinations north-south or south-north). Pickups with opposite polarity will "hum buck" or cancel out local radio noise.
Years later, Sir Brian turned the magnets in the middle pickup upside down, reversing its polarity (RP), and reversed its wiring (RW). This had the effect of making the bridge and middle pickups hum buck while in phase - one of his favourite pickup settings, as head in songs like Tie Your Mother Down and Hammer to Fall. However, this modification meant that another preferred setting, middle and neck out of phase (used in the Bohemian Rhapsody solo) was no longer hum bucking.
If Sir Brian had instead flipped the bridge pickup instead of the middle, both configurations would have been hum bucking. However, he couldn't because he had filled that pickup with araldite!
Modern pickups are wax potted to achieve the same dampening effect as Araldite without permanently sealing the pickup.
Modern Brian May Tri-Sonic style pickups have one pickup RW/RP. Some guitarists like the RW/RP pickup in the traditional middle position, while others prefer it in the bridge position.
Some pickup models (e.g., Burns BM, and Yonderbosk) reverse the RW/RP pickup's wire colours for simplicity; some (e.g., Kent Armstrong, BMG Special) use different wire colours for each pickup; others (e.g., Adeson) prefer to match Sir Brian's original set and leave the wire colours as-is.
It's important to know how the pickups are intended be wired. Reversing the polarity (RP) without also reversing the wiring (RW) means that when the phase switches are in the in-phase position the pickups are north-south but out of phase (effectively north-north or south-south and therefore not hum bucking). Reversing the wiring when it's not intended would have the same effect.