Understanding Gain Structure

Tone Secrets Without Screwdrivers: Finding Mountains Of Tone In
A Guitar-Amp Rig by Understanding Gain Structure.

(Part one…or how I learned to stop worrying and love the master volume)

By Ray Parrish

Master volume is the work of the devil…..

…unless you know what master volume actually is…..unless you know that gain is not dirt…..distortion isn’t just dirt…..saturation is only part of distortion…..that distortion means the signal coming out doesn’t look like the signal that went in…..that distortion you don’t like is noise…..and distortion you do like is signal.

Volume and master volume aren’t just about volume. They’re really where your tone lives. Most important of all, once it leaves your fingers, it’s all about the signal. What if I just want to find all the cool tones in my amp without ever using a screwdriver to open it up? What if I don’t want to build the amp……I just want to play it? Willie Johnson, I hear ya’!

No doubt there’s tons of very cool stuff hiding in any guitar/amp rig, some of it very hard to find for all but the initiated, so IMHO it really helps to know a little bit about how your rig thinks in order to find it all. By learning how amps and guitars think in general, a lot of those knobs on the front and rear panels will make gobs more sense and will greatly increase your power to sculpt tone before you ever touch an effect. So this article is the first in a series designed to show you, sans screwdriver, how to do just that with gain structure.

For starters, you need to define gain if you’re going to control for it as an amp user. Very simply, gain is how much the signal strength grows by having passed through an amplifying device like a tube or transistor. Gain is not, however the same as distortion, saturation, drive, or any other popular industry misnomers. Gain is just a small signal made bigger. It follows that each place you take the signal and amplify it is a gain stage. And each one of your amplifying tubes or transistors is, in reality, (at least) one gain stage. A guitar amplifier is made up of several such gain stages, not just one. Controlling the gain you get from each gain stage, and what it does to all those that follow, is the secret of taking control of your tone.

The takeaway here is that these are all different phenomena, and the names in proper use are not interchangeable. The trouble is, in the gear biz, we do just that all the time, especially when it comes to putting decals on control panels. But, for the intelligent knob twister, knowing the difference is what puts you in command of your sound….in the front seat instead of the back seat. So literally speaking, what can you get from any given gain stage once you have control of it? What do you expect to hear when you twist a knob? If you’re going to layer these things, one gain stage after another, it helps to know what they are.

You probably know that if you push any gain stage hard enough, it will distort simply because it runs out of capacity to accurately amplify. Distortion is any sound that wasn’t in the original signal when it went in, and it can take many forms, including noise, clipping (aka dirt), altered frequency response, additional harmonics, or a variety of other more subtle and musical effects like sag and swirl. (we’ll dig into more technical—and useful–definitions in later articles. We’re good for now.)

Further, the various types of distortion come on in a kind of a little procession, one effect at a time, more or less. Typically tubes start out by adding the harmonics that make your guitar sound big and fill out your power chords. As you turn up, the rest of the circuit gets into the act and the EQ migrates toward the mids that cut through a mix, tailing off the muddy lows and screechy-loud highs. Approaching saturation, (but still what we think of as “clean”) the signal begins to compress, giving you that soaring lead sustain, not to mention filling out the body of your notes and voicing the harmonics and EQ shift ever more clearly. Further still, you get into the actual clipping that we call dirt. At extremes, the amp tone begins to dominate the heavily clipped guitar tone until finally it’s all about the amp, with the guitar relegated to defining the notes you’re playing and the voicing of your pick. It’s also here that complex interactions like swirl, sag, and bias excursion live, all of which do cool things if you know how to use them.

We’ll explore each of these behaviors more as need be, but for what we’re about to do here, this is more than enough to inform our inventive urges on a rainy day. Now that we know a few types of distortion, and understand that they come into play in a relatively gradual and progressive onset, let’s think about how we can control all those behaviors from the front panel. To know that, here’s a strange one that ya’ really gotta know: what does a knob actually do?

Controls of a Deluxe Reverb II
Controls of a Deluxe Reverb II

Oddly enough, when you get right down to the physics, knobs can only do one thing: attenuate. Even “active” knobs cannot add anything all by themselves—that takes a gain stage with—you guessed it—an attenuator. In an amp, gain, volume, tone, and master volume knobs simply control the attenuation of the output of each gain stage as the signal gets passed along. Yup, all three are known as attenuators, and all three do the same thing–attenuate. Weird! The first time I got my mitts around that concept, it really re-wired the way I thought about what I was doing to my gear.

By the way, for the time being we’re going to refer to knobs as attenuators, just to help picture what they’re really doing. We’ll talk about the different ways to wire attenuators later, (aka pots and all that) when we have a reason to care. For now we don’t. Right now, we’re actually going to draw a picture of the gain stages in an amp and show which attenuator controls which stage—in other words, we’re making that treasure map.

With complex amps, it helps to draw yourself a block diagram so you can see where each gain stage is, and where each volume-type attenuator (aka knob) is in the circuit path. Color me dumb, but I just can’t see electricity without a picture. It’s also extremely handy to note where your tone knobs are, since they’re actually frequency-specific volume attenuators too. So, just for instance, here’s a block diagram of my knob-infested Deluxe Reverb II, a rather complex little chameleon of a beast, and one not particularly inclined to suffer fools…

Click here to continue to Part II

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