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All About Class A Amplifiers

April 1, 2008
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First, take this info-byte onboard: the tweed Fender Bassman, blackface Fender Twin Reverb, Deluxe Reverb and Super Reverb, Marshall JTM45, Mesa/Boogie MkI, Soldano SLO and many other classics are not class A amps, not by any twisted, spun, or convoluted assessment of the term. Phenomenal amps? Yep. Monsteriffic tone machines? Damn straight. So as we proceed with this investigation, hold on to the simple fact that many of the best sounding and most influential tube amps of all time are not class A amps, they are class AB amps. On the flip side, the Fender Champ (tweed, blackface, silverface), Vox AC4, Gibson GA-8, Kalamazoo Model 1, Wards Airline GDR-9012A, Silvertone 1481, and plenty of other similar practice, student, or just plain cheapo amps are most definitely class A amps, although they were not promoted as such in the trade ads of their day. And up on the “for further examination” bench, a vintage delectable such as the tweed Fender Deluxe—never touted as class A design but an all-time classic nevertheless—easily falls into the more colloquial definition of class A, as bandied about by many amp makers today. Buckle up . . .

Definitions of operating classes relevant to tube guitar amplifiers are really pretty simple. We’ll look briefly at these definitions as they apply to push-pull (PP) amps, which is the majority of amps other than the small, single-ended amps listed above. In a push-pull amp, which has two output tubes or four tubes working in two pairs, one tube (or pair of tubes) works to amplify the peaks of the audio signal (the waveform, which most of you will know looks like a rolling pattern of hills and valleys) while the other tube or tube pair amplifies the valleys. In other words, one tube pushes the hills while the other pulls the valleys, then they swap. Since AC (alternating current)—which is what passes your guitar signal from stage to stage within the amp—carries constantly alternating hills and valleys, each tube of the PP setup receives a steady stream of each. In many guitar amps, the majority in fact, each side of the PP setup actually shuts down briefly during some part of the cycle, when the other side is at its peak of current flow. What? One tube shuts down while the other amplifies? That’s right, and then they swap. Looks crazy on paper, but of course it all happens so fast that there’s no audible gap in the performance. Because the 360-degree waveforms that are being amplified by each side of the PP setup are reverse phase to each other, there’s no volume loss as one side dips and the other rises. The majority of guitar amps function in this way, including classics like all the big Marshalls, Fender Bassmans and Twins, Dumbles, Mesa/Boogies and so on.

Indeed, as we have already discovered, amps that perform as I have just described are class AB amplifiers. By definition, one side of the PP tube pair of a class AB amplifier rests for at least some portion of the cycle (when measured at maximum volume before clipping). In simple terms, that’s really all there is to the definition of class AB, or all that you need to worry about at least. It’s worth knowing too, though, that sharing the load makes output tubes configured in class AB design a little more efficient power-wise, and tends to make amps sound a little tighter, firmer, and punchier.

An output section operating in true class A, on the other hand, has the tubes working the entire cycle of the waveform (when measured at maximum volume before distortion). This is true even of push-pull amplifiers, where both tubes are sending the signal along to the output transformer together at all times, not alternately resting as with class AB. As such, class A output stages are somewhat less efficient than class AB stages, which can be driven to higher output levels. Players and amp makers often talk of sweeter distortion in class A amps, but true class A operation actually has less distortion content at a given output level. Class A distortion is smoother at the onset and is usually heard as being harmonically richer, too. But the fact that definitions of operating class are measured at maximum output before distortion should tell you something: a lot of voodoo is talked about class A—particularly by amp makers’ marketing departments eager to sell you a particular new model. The sound of true class A, operating within the realm of its definition, is actually something different than the advertising slogan “real class A tube amp” means to imply. Relatively few amps fit the definition for class A absolutely and beyond debate, which is not something to worry about at this juncture. The characteristic sound of different classic tube guitar amps is determined by far more than just their class definition.

Designers seeking true class A performance in PP amps achieve it by manipulating two factors: the DC voltage delivered to the output tubes and their bias setting. They force the tubes into this state of constant operation by carefully setting their bias point, which is a very complex matter (roughly speaking, we can equate a tube’s bias with a racecar engine’s idle). In short, class A amps are usually biased very “hot,” which—when done correctly, and for the right reasons—can make them very rich sounding but also quite inefficient in terms of output level. Such biasing is far and away most often achieved in class A amps (as well as amps purporting to be class A) by a method known as cathode biasing. This will be most familiar to guitarists in the Vox AC30 and AC15, or other amps that follow those templates. Now here’s the rub: Cathode biased amps, whether they are class A or class AB in the purest, technical sense, have a discernable sound, which itself has come to be associated with some of the supposed characteristics of an amp being class A. This sound is harmonically lush, shimmering, sometimes just a hair grainy and loose, and fairly smooth when cranked up into distortion. Often it is not particularly tight, punchy, or bold—or at least less so compared to a more efficient fixed-bias output stage. And remember, neither is better or worse than the other, it all depends on what you’re seeking to achieve with your tone.

On top of this, and to compound matters, most amps billed as class A also lack something called a negative feedback loop. Applying a little bit of an amp’s output signal in reverse phase back to the front of the output stage (the input of the phase inverter) via a network known as a negative feedback loop can help to improve the overall tightness and definition of its sound. As you can see, therefore, eliminating the negative feedback loop further contributes to some of the tonal qualities that are already being emphasized in the cathode-biased amp. You can bet your bottom dollar that almost any amp you encounter that’s billed as class A will lack such a loop, and whether or not anyone is ever going to heft it up on the workbench, attach the meter and scope, and determine whether it is truly operating in class A, such an amp will still produce what we commonly consider the class A sound, mainly because it is cathode biased and carries no negative feedback loop.

Care to guess what you’d find inside a Vox AC30? Cathode biasing and no negative feedback loop. And the same goes for amps like the Matchless DC-30, Bad Cat Black Cat 30, TopHat King Royale, Mojave Sidewinder, Dr. Z Stang Ray and others that follow the AC30 template (in addition to all the smaller amps that emulate the AC15). Other amps that fall into the cathode biased/ no negative feedback category include the ’50s-era 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; and plenty of Valco-made amps.

Toward the start of this article I listed a number of smaller practice amps that do qualify as genuine class A designs. Many of you will already have spotted that these are all what we call single-ended amps, which is to say they have just one output tube. When just one tube is working in an output stage, it is categorically operating in class A because, of course, it cannot shut down during any portion of the waveform. As employed in guitar amps, single-ended tube output stages only stray into medium-sized models (and the lower end of the category as well) in the form of dual-single- ended amps (also called parallel single ended). Dual-single-ended amps—such as the Gibson GA-8—use a pair of output tubes working in parallel to increase the potential output. Rather than working in turns like the PP pair, however, they are really acting like a single tube for operational purposes, both pushing the same signal the entire time. Modern production examples of these are extremely rare, and include the THD BiValve and Victoria Regal II, which are about the largest single-ended amps I can think of.

Even with two output tubes, such amps are still very inefficient compared to PP amps using the same tube complement. A design such as the GA-8 probably only puts out about eight to ten watts, compared to the 15-watt rating of an amp like the Gibson GA-20 or Fender Deluxe, and the Victoria Regal II’s maximum output is around 30 watts from a pair of 6L6s, which could produce 50 watts or more in an efficient class AB push-pull design. Still, it’s one way of getting a little more power out of a true class A design than the measly four watts a Champ can drum up.

In short, an amp’s class definition can provide some clues about its likely tonal palette, but the potential for misinformation and misdescription—coupled with the fact that it’s just good to get out there and sample as much gear as you can—means you should play as many amps as possible, whether they’re labeled class A or AB, before making any big leaps. Hone in on the heart of your tone according to what amplifier feels and sounds right to you and works best for your style of music, and let categories and class descriptions take a back seat to your ears and your fingers.

BIASED VIEW

The two main forms of biasing used in tube guitar amplifiers today are cathode biasing and fixed biasing. “Bias” is a term used for the way in which a tube’s operating level is set— usually with a resistor or simple network that determines how efficiently the tube uses the voltages applied to it to amplify a guitar signal. The terms for these two techniques can be misleading: fixed bias amps very often carry bias-adjustment pots, and it’s necessary to rebias them in most cases when you replace the output tubes. This is the case with all the larger Fender and Marshall amps, and others made to those templates—although early fixed-bias tweed Fenders, some Mesa/Boogies, and others have a preset bias network that is not adjustable. On the other hand, cathode biased amps most often have their bias permanently set with a resistor tied between the cathodes of the output tubes and ground. Class AB amps are usually fixed bias, and while almost all class A tube guitar amps you encounter are cathode biased, not all cathode biased amps are class A.

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