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Class A vs. Class AB

October 24, 2011

Everyone knows that that an “A” on your report card means you kicked ass in class, but is a “class A” guitar amplifier also mean it’s the best? To answer that question we first have to understand what classes mean in terms of amplifier design.

Class A

Consider a vacuum tube as an electronic valve that controls current flow much like a faucet controls the flow of water. The valve can completely cut off the flow, and when this happens with a vacuum tube, we call the condition “cutoff.” Conversely, when a valve is opened all the way to allow as much flow as is possible, this condition (in electronic terms) is called “saturation.” In a class A design, the no-signal idle current of the output tube is set (biased) halfway between cutoff and saturation (the equivalent of a faucet opened half way), and care is taken to have a small enough preamp signal driving the “control grid” so that the tube can’t be driven into cutoff, and is on all of the time. Note that since the tube can’t go into cutoff and still be operating in class A, it can’t be driven into saturation either.

Class AB

All Class AB amplifiers are operated in what is called a “push-pull” configuration. In a push-pull configuration, there are at least two output tubes driven by a phase inverter. One tube is driven in phase while the other is driven out of phase. This means one tube gets a negative signal, while the other is getting a corresponding positive signal and vice-versa. In this arrangement, one can have an input signal large enough to drive one output tube into cutoff while the other is saturating. The two output tubes drive opposite ends of the output transformer primary such that the transformer flips the phase of one output tube’s signal and sums both signals. The cool thing for guitarists is that when you add saturation and cutoff together, you still get saturation. The sound is very cello-like, with plenty of sustain and compression.

Since one tube is always resting during part of the input cycle, Class AB designs can use much higher plate voltages and much greater signal voltage, which results in much greater output. For example two 6L6 output tubes operating in Class A may yield only 10 watts, while the same two tubes operating in Class AB can provide 50 watts.

Single-ended versus Push-Pull

All amplifiers with one output tube are “single ended” designs intended to operate in Class A. However, Class A amps can also be configured as push-pull provided the tubes are idled at halfway and the input signal never drives either tube into cutoff. A Vox AC30 has four output tubes in pushpull configuration, and was designed with the intention of being a Class A. However, when a strong enough preamp signal drives the output tubes into cutoff during any part of the input cycle, then the amp is operating in Class AB by definition.

Hopefully this clears up some of the confusion about amplifier class and whether one is better than the other. But one thing to consider is that almost all of the top players—including Stevie Ray Vaughan, Jimi Hendrix, Joe Bonamassa, Billy Gibbons, Eric Johnson, Jimmie Vaughan, Buddy Guy, B.B. King, and Robin Ford—prefer class AB amps because of their overdrive characteristics.

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