You may have noticed that as you roll off volume you lose the treble end of your sound faster than the low end and you end up with a dark, muddy tone. There is a physics-related explanation, involving relative resistance and capacitance between potentiometers and pickups, but to be honest we don’t need to go into that here to get a practical understanding of the problem. Suffice to say that, as they are brought into play to reduce gain, volume potentiometers work raise resistance, creating what’s called a ‘low pass filter’ – low pitch is allowed to pass through and high pitch is filtered out of the signal to a higher degree.
This is more of an issue with single coil pickups than humbuckers and some people don’t ever notice it because they have their guitar turned up to ten all the time (that's me!). However, if it’s an issue for you, the way to resolve it is to employ a high pass filter which bypasses the volume potentiometer in the form of a treble bleed mod. As the name suggests, a high pass filter is the opposite of a low pass filter and allow through only high end pitch. This is achieved by placing the filter between the ‘in’ and ‘out’ lugs of the volume pot (the ones that aren’t connected to the back of the pot). It’s an easy mod to achieve as nothing else needs to be de-soldered.
There is a potential drawback with treble bleed, in that the guitars tone may become too bright when the volume is rolled off. But there are different varieties of treble bleeds so you can experiment with different schemes and values. There are three different treble bleed mod options (from left to right):
1. A capacitor on its own, usually of a value between 500 – 1000 picofarads
2. A capacitor of between 1 – 2 nanofarads in parallel with a resistor of a value between 100 – 350k
3. A capacitor in series with a resistor (values below)
The capacitor on its own might create too much brightness for you as you roll off volume. Adding the resistor in parallel, the Seymour Duncan scheme, will attenuate the amount of treble pitch being passed through. Option 3, sometimes called the Kinman treble bleed (after its creator Chris Kinman), employs a resistor and a capacitor in series and is in my opinion the most useful solution (Fender agree and use it in their guitars). However, you might want to use option 1, a capacitor on its own, in a bridge pickup where you want to maintain the bite of the sound as you roll off to a greater extent. PRS use a cap only in their guitars. It’s all very subjective and in the ears of the beholder, so use whichever one you perceive to be the best for your guitar.
Values of the caps and pots are important, but within reason there’s no harm in experimenting. A lower value capacitor allows more highs through and a higher value resistor prevents them from going through. Chris Kinman recommends a balance of a 1.2nF polyester capacitor and a 130k resistor and recommends that you should match a low value cap with a low value resistor (or high with high).
Despite reports to the contrary, you will find that different types of capacitors impart different qualities to tone. However, you don’t need to spend a fortune on Russian military paper-in-oil caps or ‘orange drops’ to get your sound (although in some guitars they will be the best tone caps to use). Inexpensive, low voltage film caps of any kind will usually work well (I like polystyrene). Physically larger sized caps will get in the way, make it harder to wire up the pot, and can create ground issues if they make accidental connections in an overcrowded control cavity.
Use a hot iron and heat sink (a crocodile clip or similar) when you solder in the treble bleed. It will help prevent the heat damaging the capacitor.
I hope this information is helpful in achieving your ideal sound.