Norm wrote:...
A well made pickup might not need potting as cited here:
Any guitar with a pickup that is microphonic but otherwise a good sounding pickup is a good candidate for potting. If your pickups aren't microphonic then there is little point to potting them ...
We probably should have ended the conversation there, as it seems to be the most solid advice.
Mike Detlefsen wrote:"Basically mechanical vibrations from the guitar shake the coils in the pickup. ... When you wax the coils, you kill this."
This sounds really bogus to me, as far as affecting tone. ...
You were right Mike that this person's statement is bogus
if they meant that the movement they described creates highs.
Here's what's happening:
A pickup is a coil of wire in a magnetic field (provided by the magnet(s) and extended/directed by the polepieces). Now if a magnetic field moves through/across a coil of wire, a current will be induced in the coil. Alternatively, if you physically move a coil through a magnetic field, you will also induce a current in the coil. That is, if the field is moving or if the coil is moving, it's all the same and a current is induced.
Normally, the coil doesn't move. Instead, the ferrous metal in the strings (which sit in the pickup's magnetic field) move when you pluck them, and that disturbs the pickup's magnetic field, which is the same as the field moving through the coil, and thereby creating an output from the pickup.
So how would the pickup windings move at all? As Mike noted, it would seem normal winding tension is sufficient to keep the windings from moving... and it is, as far as I can tell. The pickup would have to have very loose components/windings, and would have to be near screaming guitar amps in most cases to get any audible feedback.
That said, I once saw Elvis Costello in concert who had one of those super-cheesy 60's Japanese knock-off guitars during one song. He held the guitar top near his face and sang/screamed and the ultra-microphonic pickups amplified his voice (in a garbled manner) through a guitar amp. But that's so rare I've only seen pickups that microphonic once.
Really, reducing feedback was probably an after-thought originally. The transformers used in guitar amps work under the same principles as pickups and are usually sitting much closer to loud speakers. But most guitar amp transformers aren't potted... why? Because potting is more expensive, and is largely meant to
keep moisture out. So expensive transformers might get potted in epoxy, varnish or a tar-like substance, but are rarely wax-potted because many waxes absorb moisture over time, which defeats the main purpose of potting.
So if the pickup's sound isn't changed by feedback vs no-feedback, then what changes between potted & unpotted?
I don't know for sure, but I'll tell you what I think is most-reasonable: all coils (and all electronic wiring in general) have stray capacitance; that capacitance from one end of the coil to the other is increased when wax (or other potting material) is present over an unpotted pickup. This is like having a very small (say 10-100pF) cap bridging the two output wires of your pickup. That rolls off
some amount of high end.
There is a mathematical formula to find the capacitance of a capacitor based on its physical properties, including what material is separating the conductors that form the cap. Air results in the lowest capacitance when you change only that material separating the conductors. Wax has a higher dielectric constant, and therefore results in a higher capacitance.
However, I'm skeptical when it comes to describing how much effect this will have on any pickup's sound. Guitar cables, amp wiring, different tube types present varying amounts of stray capacitance, so any of them could have as much, or more, impact on your guitar's tone.
Additionally, any pickup maker knows there are a lot of competing variables to any pickup design, and changing any one of them can give you a darker or brighter or midrangy-er or louder or softer pickup. And your amp, speaker, control settings, effects, etc also shape the guitar's tone... so is that stray capacitance really that big of an issue?
As with everything sound-related, tone is in the ear of the beholder. You'll have to try both ways (if you're worried there's a difference) and reach your own conclusions.