What is Class A and how does it differ from Class AB?


What is Class A and how does it differ from Class AB?

The following is extracted from an Article - Class 'A' Exposed and Explained - written in 2005 by MESA Founder, Randall Smith. Click here to download and read the full article (with illustrations & examples).

The most important thing for a musician to understand is that the different amplifier Classes and Configurations serve different purposes and styles. No one is flat-out better. A player looking for maximum clean headroom will want a Class AB, push-pull amplifier because power is its forte. A player seeking a certain vintage vibe may well prefer a Class A amp, but no matter what tube is used, it will definitely be less powerful. Either type can be made to sound warm and lush or brash and biting – mostly as a result of other important factors such as the pre-amp and driver circuitry, transformer and component choices and, of course, speaker options.

Configuration refers to the number and arrangement of the tubes. The common terms for describing power tube configurations are “single ended”, “push-pull” and “parallel”. In single-ended, the entire signal runs through a single path and is amplified by one tube at a time. The much more common power configuration is “push-pull”. Here the signal is first split into two halves, 180 degrees “out of phase” with each other. The “pluses” are amplified by one tube, and the “minuses” are amplified by another tube. Then the two signal halves are “recombined” in the output transformer back into a single-ended voltage to drive the speaker.

Parallel simply means additional identically wired tubes are added to increase the power, either single-ended or push-pull. But as a practical matter, single-ended parallel is almost never used and here’s why: Even though adding a second tube in parallel with a single-ended circuit will double the power, the same two tubes wired instead for push-pull can triple the power, or more. And the reason for that difference has to do with that other term: the Classes of Operation.

The “class of operation” describes how the tube(s) (in any configuration) are “biased to operate”: are they
Class A, Class B or Class AB? (There is even a Class C although it’s used for high power radio transmission, not audio.) “Biased to operate” simply refers the various voltages on the tube(s) and how they relate to one another, especially the negative “bias” applied to the grid. These voltages determine how much electrical current flows through the tubes both when they’re amplifying and when they’re “idling”, waiting for you to play a note. Thus the “class of operation” (determined by the voltages present) is totally separate from how the tubes are arranged and every amp circuit has both a class and a configuration. Since pre-amp circuits are always run single-ended and Class A, when we speak of other classes and configurations we’re generally referring to the power sections of amplifiers where the horsepower is generated to drive the loudspeaker.

What Class A really means is that “Grid bias and alternating grid voltages are such that plate current in a
tube flows at all times”. That’s the entire definition from the RCA Tube Manual: the Ultimate Authority itself. To make it simpler, think of a tube in the British sense: It’s a “valve". All Class A means is that the valve would never shut off all the way. Some amount, even a trickle would always be flowing through it.

The Classes of Operation have just as much to do with the idle state as with the actual amplification itself. Consider Class B, because it’s easy to picture. In Class B, the bias is set so no current flows when there’s no signal. Thus no power is being consumed and no heat generated. (Think of this as “zero idle RPM”.) Then when a signal voltage hits the input grid, the positive half-cycles turn the tube ON, it begins to conduct current from the power supply directly into the load (usually a transmitting antenna) and very efficient power amplification takes place.

Then there is Class AB. This would be like having the motor go to a low RPM idle when you’re stopped. It’s still turning over (some current is flowing) but not nearly the maximum available. You could let out the clutch and putt around but only at a slow-to-moderate speed. But to get fast (or loud), you have to increase the total current flow – not just cause it to fluctuate. The loudness at which the total current begins to increase is the transition into Class B. In many ways Class AB is the best of both worlds for audio and it’s how the 6L6 Lone Star, the Rectos, Stilettos and most Fenders and Marshalls operate. Its high efficiency makes it the pinnacle of power for clean, cool audio.

Then there’s good old Class A. In a single-ended, pure Class A power circuit, the current runs at 50% of maximum even at idle when there’s no incoming signal. Then when a small signal voltage hits the input grid, it causes the current flow to fluctuate up and down between, say 60% and 40% of maximum. A louder signal would cause greater current fluctuations, say between 80% and 20%. And the maximum undistorted signal output would occur when the input signal drives the tube so it conducts a current that fluctuates between 100% and 0% at the A-440 or whatever the input signal frequency is.

Notice how the current fluctuations in a proper Class A amplifier are always centered around the mid-point, that 50% of maximum which is the same as the idle current. What this means is that there is no net increase in the current flow like there is in Class B or AB, no matter how loudly you play. In a single-ended configuration, the increases and decreases in current flow are momentary (at the signal frequency), equal and opposite around that 50% midpoint. At one instant of the A-440 there will be more current flowing, but in the next instant, there will be an equal amount less flowing. Thus the total over any period
of time remains constant.

Now in a pure Class A, Push-Pull amplifier, 100% of the maximum current flows at idle, 50% through each side. When an incoming signal causes fluctuations, the current in one side of the push-pull increases from, say 50% to 70% while current in the other side simultaneously decreases from 50% to 30%. Th e two signal halves alternately offset each other so the total current flowing through the output circuit remains the same at 100%.

In Class A, the current swings are always centered around that mid-point idle current which is 50% of maximum. This is called “biased around the mid-point of the linear region”. That’s vital
for low distortion. The other classes of operation, Class B and Class AB are definitely not biased anywhere near that mid-point and that’s the key to their ability to run cool and produce more power.

The push-pull configuration with its balanced operation makes Class AB possible. What Class AB does is fill in the “gap” in the middle of Class B push-pull operation. It eliminates the cut-off by ensuring that some current flows during idle and throughout the transition from one side of the push-pull to the other. 

In electronic terms, Class AB push-pull is like two asymmetrically biased amplifiers that mirror image each other. At idle, both are turned on around 10% to 30%, as determined by the bias setting. Because they are biased closer to OFF than ON, they run cool. They also have the potential to turn ON much more than they can turn OFF because at idle, they are nearly off already. For small signals they work just like a Class A amplifier: merely modulating the idle current flowing through them, neither turning off nor requiring an increase in input power. But as you play louder, the Class B asymmetry begins to show. Each side of the push-pull alternately turns way more ON than off , causing the current to increase first through one side then through the other. And as the opposite side – the one with the diminishing current – approaches cut-off , the ”on-going” side has already entered its linear region and can take over while avoiding the distortion around cut-off . Providing such a Class A zone of overlap between the push and pull halves of a Class B amplifier allows the transition to occur smoothly by eliminating the “dead spot” in the middle. And the reduction of wasted dissipation is huge.

Since we’ve reduced the idle current (amps) by biasing the tube away from that midpoint, we can now substantially increase the voltage and still keep the dissipation at a level well below what it was for Class A. And as the voltage on the tubes increases, so does the amplitude or amount of clean power they can conduct. It can go way up.

In power output stages, what usually happens before the power supply runs out is that the power tubes themselves reach maximum capacity, literally running out of room on the insides of their plates. So much current is flowing that the entire inner surface is saturated and the extra electrons arriving there have no place to “stick”. This is why we can raise or lower the output capacity of the amplifier by switching additional tubes in or out. Authentic retro tone is all about power and the way it clips. Yet in any given amp, the sweet-spot of power clip is inseparably tied to its wattage and playing loudness. Our two Lone Stars (Lone Star & Lone Star Special) for example, smash this age-old limitation by giving you distinct and switchable power configurations, each with its own window of wattage for genuine power tube clip, beginning at 5, 15, 30, 50 or 100 watts, depending on the amp and its setting. And these are channel-assignable, so each can be tailored for the tone and power you want.

There are a lot more details, examples and illustrations in Randy's Article, "Class 'A' Exposed & Explained". Click here to download and read the full article.

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