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What's the Big Deal About Class-A Amps?

January 10, 2014
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THE TERM “CLASS A” HAS HAD a major buzz in the amp world for many years now, and simply on the face of it would seem to imply the best and most desirable. Compared to the other major category of tube amp, class A/B, class A has simply got to be better, right? I mean, if you were throwing together a killer steak dinner, you wouldn’t prefer grade A/B beef over grade A, right?

To get anywhere close to answering this, we need to consider two major caveats. First, when it comes to tube amp classifications, things don’t work in the standard “A = best” way. Put simply, the class assigned to any particular tube circuit defines the way in which the tubes function, not the quality of the tone that comes out of them. Second, amp manufacturers describing their products as “class A” aren’t always doing so according to the strict laboratory sense of that classification, which further muddies the waters for any guitarist seeking tonal distinctions.

Both of these are addressed, to a great extent, in a better understanding of how amplifier classes are defined. A class-A amp is one in which the output tubes are functioning throughout the entire cycle of the waveform (the AC signal that carries the guitar sound) without shutting off at any point, when measured at peak output before distortion. Class A/B amps are defined as those in which alternate output tubes shut down for a brief part of the cycle while the other tube or tubes take over the job of amplifying the signal.

In both of the above, we’re talking “push-pull” amps—those in which one tube out of a pair, or two out of a quartet, pushes the hills of the waveform while the other pulls the valleys. You might not think any output tube could shut down mid flow, but this happens so quickly that it isn’t audible to the human ear and is merely a component of how the circuit works. This “how it works” is, however, a factor in its overall sound. If this explanation of amp class makes class A/B amps sound even more inferior, consider this: Marshall 50- and 100-watt models, Fender tweed Bassman, Deluxe Reverb, Twin Reverb, and Super Reverb; Mesa/Boogie MkIIC+; Soldano SLO, and many more classics are class A/B amps.

The confusing part of all this is that several amps that are often described as being class A—we think of Vox, Matchless, Bad Cat, 65amps, TopHat, and several others—will actually display class A/B tendencies when put up on the bench and measured in laboratory conditions. Where we do hear very legitimate differences between many amps by these makers and those in the class-A/B list above, though, is in two significant factors shared by most amps billed as “class A”: they are cathode biased, and they have no negative feedback. Both of these contribute to a looser, juicier feel and tone, particularly at the edge of distortion. Add the fact that these are also frequently amps that use EL84 output tubes, and you get closer to some verifiable reasons behind their sweet, chimey, articulate sound and harmonically complex breakup.

So how does the class-A or class-A/B distinction ultimately figure into your tone quest? Strictly speaking, perhaps it shouldn’t. Look at it more from this perspective: If you groove on punch, clean headroom, and/or super-firm lows, you might look more toward traditional class-A/B amps (aka fixed-bias amps, with a little negative feedback in the output stage). If, however, you want emphasis on chime and shimmer and some extra harmonic content in your overdrive tones, examine the cathode-biased amps that are often promoted as class A.

Either way, don’t let the tags and categorizations lead your search. Use your ears and your fingers, and go with the tone that inspires you.

SINGLE-ENDED AMPS: THE REAL CLASS-A

There are a number of irrefutably class- A amps out there, and you’ll find them among any of the several single-ended designs that have proliferated in recent years. These are all descendants of little practice or “student” amps of old, such as the Fender Champ and Vox AC4 and the like, low-wattage amps with just a single output tube. With only one tube doing all the amplification work, these single-ended amps are truly class A out of necessity: They can’t shut down for any portion of the waveform. Some amps with two tubes working in series, known as “dual-single-ended” amps, are also class A, since the two tubes are really working together as one, rather than working at opposite poles of the waveform in a push-pull configuration. So, if you want to hear an example of unquestionably bona fide class-A tone, plug into something with a single output tube and crank it up. —DH

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