Fat, bright, warm, thick, twangy, jangly, hot, creamy—guitarists use a lot of adjectives to describe their amps’ tones, and the longer you play, the more you come to understand the gist of what these terms represent sonically. But what’s responsible for these foundational characteristics? What is it within a circuit that makes one amp sweetly clean while another is raw and crunchy, when both are set to the same position on the volume knob?
There are several factors under the hood that affect the aural characteristics of the signal that passes through your amp, but many of the ingredients that make various classic tube amps so different from each other are found within their gain stages. On top of that, the amps’ tonal traits are further determined by how these gain stages interact with each other within the circuit as a whole—for example, how the first gain stage impacts the second, and vice-versa, as the guitar signal makes its way down the line. Even beyond the ratios of clean-to-mean and hot-to-cool that are determined by an amp’s gain stages, the various configurations are also responsible for a huge part of its voice—its core tone. In most amps, the EQ stages that we often think of as shaping the tone are there just to take away specific frequency bands from the sonic foundation that has already been formed elsewhere.
Looking beyond that core voice, gain stages are what make some amps shimmer while others scream. The Mesa/Boogie Mark IIC+, Fender Twin Reverb, Marshall JCM800 2203, Bogner Ecstasy, and Soldano SLO are all tube amps that run at approximately 100 watts, but their very different types and numbers of gain stages are responsible for making one wail, another crunch and thump, and another chime and ring. Let’s dig into the basic building blocks of different gain stages found in several classic amps’ preamp sections, and then we’ll see how coupling them to a range of different output stages further shapes their tone and response.
Setting the Stage
“Gain stage” is the term used to describe any place within a guitar amp where gain is added to the signal—that is, where its strength is increased. The external clue to a gain stage is often found by the presence of a knob that makes the amp louder in one way or another. It might be labeled volume, gain, drive, overdrive, lead, rhythm, or something else, but if the amp gets louder (and/or more distorted) when you turn it up, chances are the potentiometer behind that knob is interacting with a gain stage.
A look at the schematic for the early-’80s Mesa/Boogie Mark IIC reveals the several gain stages, related level controls, and the tone stage in the complex preamp envisioned by company founder Randall Smith.
In traditional all-tube guitar amps, gain is achieved with tubes, so each of these gain stages we’re discussing revolves around a specific part of the circuit dedicated to helping each preamp tube do its thing. In fact, there are two triodes within each of the most common preamp tubes used in modern guitar amps—12AX7s and the like—so each of those little tubes can be wired up as if it is two preamp tubes within one bottle. In other words, the most common preamp tubes can provide two gain stages. (That said, some more esoteric amp designs use an EF86, a pentode preamp tube popularized by vintage Vox AC15s and modern Matchless DC30s alike, and which has only one gain stage per tube but is capable of more gain than a 12AX7.)
Of course, a12AX7—and related tubes like the 12AT7 and 12AY7—can also be used to perform other functions within the circuit, including as part of the tremolo, reverb, or effects-loop circuit, or as a phase inverter, for example. All this means that simply counting your amp’s preamp tubes and multiplying by two doesn’t necessarily reveal its total number of gain stages.
As complex as it is, the single-channel circuit of the Matchless John Jorgenson Signature Model requires only one gain stage, thanks to the capabilities of its EF86 pentode preamp tube (not shown), which is mounted on the other side of the chassis behind the associated circuitry highlighted here. The EF86 is also used in the Matchless DC30.
The thing about gain stages is that their effect on any amp’s overall sound is both micro and macro. That is, the accompanying circuitry (resistors, capacitors, etc.) deployed by the amp’s designer plays a key role in further shaping the tonal character of each gain stage. But that sound shaping is also cumulative.
So when several gain stages are involved—and there are at least two in the preamps of most guitar amps—each begins to act upon the other to determine the sound of the guitar signal that comes out the other end. As we’ll see below, the amount by which multiple gain stages increase the guitar signal as one chains into the other in a more complex multi-stage preamp also determines how much distortion can be achieved when you turn up a high-gain amp.