You've probably experienced this at some time; you crank up the gain and your eardrums are pierced by a shrill uncontrollable squealing. This is feedback, but not the controllable feedback that allows you long sustained soloing; this is microphonic feedback.
On an electric guitar most microphonic feedback is caused by one of two things.
The primary cause are the windings of the coil in the pickup. These are wound under tension and can vibrate at high gain. Immersed as they are in the magnetic field of the pickup's core they induce a series of random signals which manifest as high frequency squeal.
A secondary cause affects covered humbuckers. Because the chrome cover of a humbucker is connected to earth it effectively presents a capacitance value across the signal path and earth. Vibration of the cover causes fluctuations in air density in the gap between the coils and the cover causing proportional changes in the capacitance seen by the output of the pickup, acting in a similar way to a condenser microphone. This can cause squeal as well, although it is less common.
The remedy for both types is to immerse the pickup in a special formulation of wax. this is done under high vacuum to ensure that all the air is drawn out of the coil. A porous tape is used to seal the coil prior to waxing. This is permeable to the air contained in the coil but once saturated with wax will not allow air to re-enter the coil once atmospheric pressure is returned.
Many people will have you believe that a pickup can simply be dipped in hot wax and soaked for a while but this offers no cast iron guarantee of success.
In my workshop I have a custom built vacuum chamber and high-vacuum pump capable of drawing 28" of mercury. The wax I use is a formulation of anhydrous lanolin and dermatological grade paraffin wax which has a similar glue like consistency to ear wax and is acoustically inert. Provided the pickup coil has not been lacquer or resin sealed or waxed previously by an amateur using ordinary candle wax (which can cause penetration problems for the correct formulation) I find I have a 100% success rate in curing microphonic squeal.
