This topic seems to come up quite often, so I figured I’d lay out my theory on how to avoid muddy sounds, and get clean, crisp distortion, and how this has worked for me.
A basic thing to know about distortion is that it’s basically a side-effect of pushing a valve amplifier beyond its limits. It results both in compressing the original sound, and adding extra harmonics on top of the root frequency (more info on that here: http://www.blackstoneappliances.com/dist101.html).
Another thing is that the frequencies in a signal are pretty much of a logarithmic nature… that is, the signal strength of bass frequencies is generally much higher than that of the higher frequencies, so the low frequencies are more likely to distort and compress.
Now the thing is… when you have too much distortion on your bass frequencies, you are adding many higher frequencies which start to interfere with the other higher frequencies in your signal, so you can no longer discern the individual notes, the sound becomes muffled and the compression will take out the ‘punch’ of the sound. I think this is what most of us know as ‘muddy’.
The solution then, seems to be that you want to take away some of those bass frequencies before it starts distorting, so you ‘free up’ some room for the other frequencies, and balance out your tone.
There are various ways to do this. One example is the Mesa Boogie Mark IV amplifier, which has two equalizers, one before the major distortion circuitry of the amp, and one after. With the first you can ‘shape’ the distortion, dial out excess bass and tighten up the sound… with the second you can make the distorted sound aurally pleasurable.
Many amps (such as Marshalls and derivatives) only have an eq after the distortion, so you need to add the ‘shaping’ control externally.
You could use an equalizer pedal, or a booster pedal, which emphasizes the treble frequencies (such as what Brian May uses, and the Ibanez TS-9 is also famous as a booster pedal… they set the gain low, and the tone and output high, making it essentially a treble booster pedal)… Or you could change your pickups to pickups with more treble and/or less bass.
I personally choose not to go for extreme pickups, because while they may make your distorted sounds sound good, they generally don’t work that well for clean or lo-gain sounds. I don’t like the compromise.
I find that with most good pickups, a treble booster is good enough to get a tight, punchy distortion sound. And for clean or lo-gain sounds you just switch the booster off, and you have your classic tones back.
I tend to cringe when people say Les Pauls or their PAF-like pickups are horribly muddy… The truth is that it’s not in the pickups.
Good PAF-like pickups have an almost singlecoil-like clarity to them, and just like single-coils, they make the sound of the guitar and your playing sound through. That’s why people love them.
So, if you have a guitar that sounds great clean, but you’re having trouble getting rid of the muddy sound with hi-gain… try boosting that treble, it might just work.
You’ll probably also find that with boosted treble, the gain doesn’t need to be that high to still get great sustain and harmonics on the high strings (which is probably why most people want the gain… to get into that screaming harmonic zone).
If you look at Eddie van Halen’s Frankenstrat… in a way he did the same… He used a PAF pickup from his old Gibson, and put it in a Strat with a Floyd and a maple neck… a very bright-sounding guitar. He got an extra bit of high-end because he removed the tone-knob. The way the tone-circuit is constructed on a guitar, it is a low-pass filter, where less resistance means more signal is filtered off. With the knob at ’10’, it is set to maximum resistance, which means there is still some signal being filtered off (depending on the resistance of the pot used). Not having a tone-knob means you get a brighter sound. So in a way that was his treble boost. The result was legendary.