Few subjects in the tube amp world have inspired as much confusion, argument, and misleading marketing nonsense as amplifier operating classes, namely what amps can truly be called "Class A." Class A amplifiers are often spoken of in hushed tones among valve tone enthusiasts, revered for their elegant simplicity, smooth onset of breakup, and the richly saturated sounds that emanate from their speakers. The problem is, few players actually have a clear idea of what Class A means, and marketing people in the M.I. industry have been more than willing to take advantage of this widespread ignorance to move product, further exacerbating the confusion.
So, what is Class A? Well, it can get as complicated as you want to make it, but the simplest answer is that, in a tube amp operating in Class A, all output tubes work steadily over the entire waveform cycle. This is opposed to the more common Class AB amp operation, where the output tubes take turns sharing the load, with half of the tubes shutting down and resting at some point during the cycle, while the other half takes over the work. You may have heard this back-and-forth concept referred to as "push-pull" operation. Where it begins getting really complicated is that some amps only operate in Class A some of the time. For instance, many amplifiers that have long been touted as being true Class A, like the Vox AC30, only operate as such when running clean; as soon as they begin breaking up, they switch to Class AB operation. The only valve-based amplifiers that truly, definitely, without a shadow of a doubt, operate in Class A at all times, are so-called "single-ended" amps, meaning that they only have a single output tube. Obviously, this tube has no partner with which to share its work load, so it must operate at full tilt 100 percent of the time. There are also the much less-common dual single-ended amps, which are designed with two output tubes that run parallel in an independent fashion, without sharing the work between them, resulting in significantly more output than most single-ended amps are capable of.
The characteristics of amplifiers that operate in Class A vary, but there are certain commonalities that they all tend to exhibit. Firstly, because Class A designs are inherently inefficient, these amps are always lower wattage, with about 35 watts or so being a practical upper limit, and four or five watts being much more common. Secondly, they typically are cathode biased and lack a negative feedback loop, which makes them a little noisier, a little looser in the low end, and perhaps a bit more wild and hairy when overdriven. They also tend to produce a saturated tone that is uncommonly rich in upper harmonic frequencies. These tonal qualities are what players have always loved about Class A amps, and why so many continue to seek them out.
Class A designs were more common in the early days of amplified guitars, with smaller student or practice oriented combos like the Vox AC4, Fender Champ, and Gibson Gibsonette being well known vintage examples. Some of these are still available today in some form, and a handful of innovative new designs can be had as well. For those with a lust for that untamed Class A tone, here are a few of our favorite modern examples of the breed.
Epiphone Valve Junior
Epiphone's single-ended, Class A Valve Jr. line was first introduced in 2005, and though the line has since been discontinued, these amps have remained popular, ubiquitous, and best of all, really cheap. The main reason for their popularity (besides the aforementioned cheapness) is their simple, reliable design, classic tone, and the great ease with which they can be modified. An internet search will reveal endless modifications for these amps, as well as pre-assembled mod kits with all the necessary parts and instructions. Modded or not, you can't afford to not have a Valve Jr. This EL84-powered amp puts out a surprisingly robust five watts in a single-channel, single-ended, genuine Class A, cathode bias design, and used heads and combos sell regularly for 100 to 150 bucks used, making it an incredible steal.
THD BiValve
Nope, it ain't no clam. In the case of this groundbreaking Class A head, "BiValve" means dual single-ended tube output, rather than a class of mollusks housed in a hinged, two-part shell. THD's BiValve is the ultimate modern Class A amp. Its design consists of two power tubes working independently in parallel, combined through a custom output transformer. This clever arrangement allows the BiValve to operate purely in Class A at all times with a comparatively hefty output of up to 30 watts. But wait, there's more! It can operate on just about any power and preamp tubes in any combination without being re-biased. So, not only can you swap tubes at will, but you can also experiment freely with odd combos, like one KT88 and one 6L6, or one EL34 and one 6V6, making the BiValve a dream come true for tone tweakers. It also features a number of other handy functionalities, including a built-in THD Hotplate attenuator, a Hi-Lo power switch, and a balanced, transformer-isolated line out with a built-in dummy load for disconnecting the speakers for direct recording. Also, it sounds bloody magnificent, achieving everything from beautiful, bell-like cleans and chiming breakup to seething saturation at any volume level.
Mojotone Tweed Champ Style Kit
Fender's Tweed Champ is a genuine Class A classic, and one of the most widely revered recording amps in history. Its single-ended, five-watt, 6V6-based circuit generates incredibly lively, dynamic overdriven tones at completely manageable volumes, endearing it to both guitar players and recording engineers alike. Vintage models are expensive enough to be out of the question for most players, and though many fine modern boutique Champ clones are available, these can often be rather pricey as well. This is kind of ridiculous considering how basic the Champ's circuit is. The perfect solution to this problem is to just build your own. For around 500 bucks, Mojotone will give you everything you need to build your own Champ combo in about four hours, and the build itself is about as easy as it gets—no prior experience is necessary. This is the perfect beginner’s amp building project, and the end result is a compact, gorgeous, Class A overdrive machine that will delight you for decades to come.
Victoria Regal
Naperville, Illinois's own Victoria Amplifier Company came to prominence as builders of vintage Fender tweed amp clones, so of course they make a heck of a Class A tweed Champ copy. That's not what I'm here to discuss, however, as Victoria also makes some original designs that are much more interesting. One of these is the Regal, which is a dual single-ended Class A combo that can operate on either one or two eight-pin power tubes, including 6V6, EL34, KT66, and more, in any combination, with output from five to 35 watts. With these capabilities, the Regal is a playground for vintage tone lovers, generating sounds that range from warm and jazzy, to raunchy old-school grind. Combining different types of tubes can result in a variety of classic American and British amp tones, as well as unique sounds that exhibit some of the distinctive characteristics of both. All of these sweet sounds are pushed through a custom Eminence-built Victoria 15-inch speaker for big, firm low end and rich upper harmonics in equal measure.
Vox AC4TV
Vox's AC4 is another tiny Class A combo, like the Fender Tweed, that was originally sold as an affordable practice amp for students, but which has now become a venerated classic for getting righteous tube overdrive tones at modest, easy to record volumes. Hunting down a vintage AC4 isn't necessary however, as Vox has wisely reissued this amp as the AC4TV. Its single ended design uses one EL84, and because its four watts of output can be surprisingly blaring, it also offers a built-in attenuator to take the power down to one watt, or even one quarter of a watt, making this diminutive combo amp quite ideal for practice and studio duties. Its voice is rich and chimey as you'd expect, and when cranked it conjures up much the same kind of grime as its bigger brothers, the AC15 and AC30. Like the Epiphone Valve Junior, the AC4TV is so good and so affordable, at around 300 bucks new (even less used!), that there's no good reason not to own one. If you're feeling fancy and have money burning a hole in your pocket, there's also a hand-wired AC4 that runs around 800 dollars new, but most players will find the standard AC4TV to be the better option for Class A tone on a working guitarist's budget.