The notion of a "Class A" amp first became a hot topic in the guitar world at large with the arrival of Matchless Amplifiers in 1989 as one of the premier makers on a burgeoning scene of so-called "boutique" amps. The choice of many hard-touring pros then and now, Matchless billed its amps as resolutely Class A, to a point that became almost a religion for the company's co-founders, Mark Sampson and Rick Perrotta.
As Sampson, Matchless' former chief designer, told me regarding Class A amps a few years after he'd moved on from the company, "For guitar amps I think they're the best. They give the best harmonic content."
Not that Matchless was first in singing the praises of the Class A tone. Randall Smith started putting Simul-Class switching in his Mesa/Boogie amps from the inception of the Mark IIC+ in 1983—which enabled the user to select between Class A, Class AB or a blend of the two. Countless other amps before it had been made to what was ostensibly the Class A format, they just hadn't been billed as such.
In the years since, makers such as TopHat, 65 Amps, Morgan, Carr, Orange, Budda, Victoria, and Dr. Z have all included amps in their lineups promoted as "Class A" (although most of these companies also build amps of "the other class" too). And of course Vox, whose golden era amplifiers are often cited as the origination and epitome of Class A amps, have continued to make them.
The thing to remember with all of this, though, is that while the name of that amplifier operating classification might imply "the best," that's not at all what the term "Class A" was intended to signify. And with all due respect to Sampson's "they're the best" comment, it's worth noting that countless legendary guitarists—and the classic recordings they've made—would beg to differ.
What "Class A" Really Means
On its surface, the term "Class A" implies "best," "top-of-the-line," and so on, like "Grade-A beef" or "first-class ticket." After all, you wouldn't buy Grade-B beef for your family, right? So who wants Class B tone? But that's not at all how this class thing works. And just to be clear, while you really don't want Class B tone (which exhibits high crossover distortion—not a good thing), many of the world's greatest players have been using "Class AB" amps for years, and such designs constitute some of the enduring classics.
Quick sidebar: Before diving into the simple definitions below, consider that the guitar signal passing through your amp either is "single-ended," meaning it runs through one path and one single output tube, or has been split into two opposite-phase strands as it enters the amp's output stage. In this second configuration, called "push-pull," those strands are amplified by one of the two output tubes (or one of two pairs, if it's a bigger amp with four output tubes). One tube "pushes" the "hills" in its waveform, while the other "pulls" the valleys in its reverse-phase waveform. The two are put together in the output transformer to send an amplified signal to the speaker.
With that in mind, by definition Class A means that a single-ended amp has its one output tube working at 100 percent at all times, or when neither of the two output tubes in a "push-pull" setup fully shuts off during any part of the waveform. This operational state is achieved through a combination of the way the tubes are biased and the voltage levels at which they are run (what is often called a "hot" bias). This makes for less efficiency, meaning less volume, but minimal crossover distortion, as well other sonic characteristics (more on that below).
In Class B, each tube conducts (amplifies) for only half of the waveform cycle, essentially shutting down for the other half. This makes for an efficient—i.e. loud—amplifier, since the output tubes are splitting the work between them, but the sonic characteristics are undesirable for guitar because nasty distortion occurs during that transition between one tube shutting down and the other amplifying.
Class AB, on the other hand, combines the characteristics of Class A and Class B, resulting in an amplifier that is reasonably efficient but also avoids nasty crossover distortion, thanks to a smoother handover of amplification duties between the hills and valleys of the waveform. All else being equal, Class AB amps are generally more powerful than Class A amps, and are also somewhat tighter and punchier sounding, usually with firmer lows too.
So what we see here is that "Class A" is not an adjective, not a descriptor, but simply a term that identifies the type of output stage used in a particular amplifier. And it's worth knowing that those class designations go further, to include Class C and Class D, which have other specific operation characteristics that we don't need to go into here.
Second Class Citizens?
As for great Class AB guitar amplifiers, how about Fender's tweed Bassman, Pro, Super, and Twin, and the blackface Bassman for that matter, as well as the Deluxe Reverb, Twin Reverb, Super Reverb and everything in between? Or Marshall's JTM45, Plexi, and JCM800 models, or Hiwatt's legendary stacks, or the original two iterations of the Mesa/Boogie Mark series, the Mark I and Mark II, or the Soldano SLO or Bogner Shiva and many more? Not too shabby, right?
Put another way, if Jimi Hendrix, Eric Clapton, Jimmy Page, Pete Townshend, Stevie Ray Vaughan, Joe Bonamassa, and countless others all did much of their most notable work using Class AB amps, well, the "Class A is best" argument kind of falls apart, right? But these two designations—Class A and Class AB—can still tell you a lot about the sonic characteristics of an amplifier, and each classification definitely has its fans, and its applications.
But here's another wrench in the works: Add to all this the fact that many amps that are billed as "Class A" really aren't Class A when examined under strict conditions, and things get a little more confusing. But hey, this is all pretty easily cleared up.
What Makers Really Mean When They Say "Class A"
Rather than immediately assuming a guitar amp strictly fits the textbook definition of Class A every time you see a manufacturer use the term as part of their promotional material, look at it like this instead: It's very hard to know whether any supposed Class A amp is actually functioning according to the strict definition of Class A without getting it up on the bench and monitoring it very carefully through the range of its operation using appropriate meters. So what that maker is telling you instead is that the amplifier is cathode biased, with no negative feedback.
Cathode biased means the output tubes are biased by a large power resistor that's permanently connected to the tubes' cathodes. No negative feedback means there's no feedback loop from the output of the stage back to the beginning of the stage at the entry to the phase inverter, something used to tighten up and clean up many other types of amplifiers, most notably many Class AB designs, but which suppresses some of the harmonic overtones in the process.
These two techniques are used in many amps that might classify as Class A when measured on the workbench. But in and of themselves they lead to the sonic characteristics that many makers and guitarists alike are referring to when they talk about "Class A amps": a high level of harmonic overtone content and a smooth onset of distortion when the amp moves from clean into clipping. Or, put more colloquially, "a lot of chime, swirl, and bloom."
Class AB amps, on the other hand, are relatively tighter and punchier sounding, with what we might hear as a crisper and more bell-like clean tone, and a distortion that's sometimes a little more jagged and eviscerating, although still smoother—in a well-designed amp—than the gnarly and unpleasant crossover distortion of a Class B amp, thanks to the blending of Class A and Class B characteristics.
What Good Is All of This?
That's a reasonable question. Or, put another way, "how does my understanding of what's going on inside any supposed Class A amp help me to be a better-sounding guitarist?"
In and of itself, it doesn't. But it just might serve several useful purposes in your tone quest by:
Letting you know that an amp billed as Class A isn't necessarily better than other amps its size—it's just describing the way the output stage is configured.
Telling you that the so-called Class A amp (which is to say, more accurately, it is cathode biased with no negative feedback) is likely to be of the chimey, swirly, saturated-harmonic Vox AC30 camp and other similar amps.
Or telling you that you might rather have a resolutely Class AB amp if you're looking for big, firm Twin Reverb-like twang, British-stack-like crunch, or eviscerating metal tones.
Finally, it's worth noting that many classic amps of old such as Fender's tweed Deluxe, Gibson's GA-40 Les Paul Amp, and Supro's Model 1624T (aka Model 24) were cathode biased with no negative feedback. Makers today would likely declare them to be "Class A," but upon their release, they were never promoted as such, simply because… no one cared. If they sounded good, they sounded good. And that's all we really need to know about them.