Class AB

Class AB is the most common, so we will start with it first.

In a push-pull (PP) amp, two output tubes—or two pairs of tubes—share the job of amplifying the guitar signal.

Think of the signal as a wave made up of hills and valleys. One tube handles the “hills,” while the other handles the “valleys.” In other words, one tube “pushes” while the other “pulls.”

The guitar signal is alternating current (AC), so the waveform is constantly moving between positive and negative. The phase inverter splits that signal into two opposite-phase versions and sends one to each side of the push-pull output stage.

Here’s where things get interesting: in most guitar amps, each side actually shuts off briefly during part of the cycle while the other side is doing most of the work. Then they switch jobs.

That might sound like it would create a gap in the sound, but it happens so quickly that you don’t hear one. The two sides are working on opposite halves of the waveform, so as one side drops down, the other rises to take over.

This is how the majority of classic guitar amps work, including big Marshalls, Fender Bassmans and Twins, Dumbles, Mesa/Boogies, and many others.

That’s basically what class AB means: at maximum output just before clipping, each side of the push-pull output stage rests for at least part of the cycle.

There are some practical advantages to this arrangement. Because the tubes share the workload, class AB is more power-efficient and can produce more output power. It also tends to give an amp a tighter, firmer, punchier feel.

Class A

A true class A output stage works differently. The output tubes conduct throughout the entire waveform cycle, even at maximum output before distortion.

This can happen in a push-pull amp, too. One tube still “pushes the hills” while the other “pulls the valleys,” but neither tube takes a break. Both are working continuously.

The tradeoff is efficiency. Because the tubes are always working, a class-A output stage generally produces less power for a given set of tubes than a class-AB stage.

You’ll often hear people say that class-A amps have “sweeter” distortion. There’s some truth behind the reputation, but it needs a little explanation.

A true class-A amplifier actually has less distortion at a given output level than a class-AB amplifier. When it eventually does distort, however, the transition into distortion tends to be smoother and can produce a rich mix of harmonics. That’s part of where the “sweet” or “harmonically rich” reputation comes from.

The important thing to remember is that “class A” doesn't automatically equal a particular sound.

In fact, relatively few guitar amps meet the strict technical definition of class A beyond any argument. And an amp’s sound depends on far more than its operating class.

So when an amp is advertised as “a real class-A tube amp,” it’s worth taking the claim with a grain of salt. Marketing departments have been known to make the term sound more magical than it really is.

Also, remember that Class A power tubes work pretty hard, so tube life is usually shorter than in Class AB.

Voltage vs. Bias

Getting a push-pull amp to operate in true class A involves carefully balancing two things: the DC voltage supplied to the output tubes and their bias.

Bias is a little like the idle setting on a car engine. It determines where the tube is operating when no signal is being applied.

To keep both tubes working throughout the entire waveform, a class-A amp is generally biased relatively “hot.” In simple terms, the tubes are working hard even when you're not asking the amp for maximum power.

When done properly, this can contribute to a rich, harmonically complex sound. The downside is that the amp isn't as efficient, so you get less output power from the tubes.

Class-A amps—and many amps that are marketed as class A—often use something called cathode biasing.

Guitarists will probably recognize this approach from amps such as the Vox AC15 and AC30 and designs based on them.

But here's an important distinction: cathode bias and class A are not the same thing.

An amp can be cathode biased and still technically operate in class AB.

Cathode-biased amps do, however, tend to have some characteristics that people associate with the “class-A sound.” They can sound harmonically rich, shimmering, smooth, and sometimes a little loose or grainy when pushed into distortion.

Compared with a more efficient fixed-bias design, they may sound less tight, punchy, or forceful.

Neither approach is inherently better. They're simply different ways of designing an amp, and the right one depends on the sound and feel you're after.

Negative Feedback

There's another design choice that often gets mixed into the whole “class-A sound” discussion: negative feedback.

A negative-feedback loop takes a small portion of the amp's output signal and feeds it back into an earlier part of the circuit, usually around the phase inverter.

Because the signal is fed back in reverse phase, it can reduce certain kinds of distortion and help make the amp sound tighter, cleaner, and more controlled.

Many amps marketed as “class A” don't use negative feedback.

Leaving it out can contribute to the looser, more open, harmonically rich character that people often associate with class-A amps.

So when you look inside a Vox AC30, for example, you'll find two things that help explain its familiar sound: cathode biasing and no negative feedback loop.

The same basic approach appears in amps such as the Matchless DC30, Bad Cat Black Cat 30, TopHat King Royale, Mojave Sidewinder, Dr. Z Stangray, and other amps based on the AC30 design.

And this isn't a modern invention. Plenty of older amps used similar designs, including the 1950s Fender Deluxe and Gibson GA-20, GA-30, and GA-40 Les Paul, early-'60s Selmer Selectortone, late-'60s Traynor YBA-2 Bass Mate and WEM Dominator, plus many Valco-made amps.

Single-Ended Amps

Some smaller guitar amps really are class A by definition.

These are usually single-ended amps, meaning they have just one output tube.

With only one output tube, there's nobody else to take over part of the waveform. The tube has to work continuously, so a single-ended output stage is necessarily class A.

The downside is efficiency. One output tube can't produce a huge amount of power in this kind of circuit, even if it's a large tube.

There are also dual-single-ended amps, which use two output tubes wired in parallel. Instead of taking turns like a push-pull pair, the two tubes essentially act like one big tube. Both amplify the same signal continuously.

The Gibson GA-8 is one example, while modern examples are rare. The THD BiValve and Victoria Regal II are a couple of examples.

Even with two tubes, single-ended designs are much less efficient than push-pull designs using the same tubes.

For example, a Gibson GA-8 might produce roughly 8–10 watts, while a push-pull Gibson GA-20 or Fender Deluxe using a similar number of tubes can produce around 15–18 watts.

The Victoria Regal II can get around 30 watts from a pair of EL34s. A more efficient class-AB push-pull design using the same kind of tube complement could potentially produce twice that or more.

So single-ended designs aren't a particularly efficient way to make power. But they are one way to get more than a few watts from a genuinely class-A output stage.

Class Discussion

So what should you actually take away from all of this?

An amp's class can give you some clues about how it works and possibly about its general tonal character. But it doesn't tell you exactly what the amp will sound or feel like.

Terms such as “class A,” “class AB,” “cathode biased,” and “negative feedback” describe specific parts of an amp's design. They don't tell the whole story.

And because “class A” has become such a powerful marketing term, it's especially easy for the label to create expectations that don't necessarily match what's happening inside the amp.

The best approach is to listen to the amp itself.

Pay attention to how it responds under your fingers, how it feels when you dig in, how it cleans up, how it breaks up, and whether it works for the kind of music you play.

In the end, your ears and your hands are more useful than the label on the amp.