Very few guitar amplifiers are strict Class A. If you have a guitar amp with one power tube, which is known as single-ended, it’s Class A by default. This would include all your standard Fender Champ type circuits, the Epiphone Valve Junior, the Vox AC4, the BlackHeart Little Giant, etc… Many amps built by Vox, Crate, Matchless, etc. are advertised as Class A but in fact are Class A/B. Now this does not mean they can’t be great sounding amps. The text below simply a technical breakdown of the operating class descriptions.
Single Ended describes an output stage that’s compromised of one power tube. There can also be additional tubes added in parallel which is often referred to as parallel-single ended. Either way this topology is Class A by default. An awesome sounding example of this is the Dr.Z Z-Plus which uses two paralleled 6V6’s.
Push Pull describes an amplifier in which the output stage is divided into two halves that are driven out of phase relative to each other. These out of phase signals are combined by the output transformer to provide one in-phase output signal that’s a composite of the original two out of phase signals. Push Pull can be designed to operate in any class but is mostly used for A/B and B.
To better illustrate the difference between singled ended and push/pull lets imagine a lumberjack sawing a log. If the lumberjack is working alone he pushes and pulls the saw through its entire range of motion, or let’s say one full 360 degree revolution. He is working non-stop and any effort being put into the saw comes solely from him. This is your classic Fender Champ with a single 6V6 power tube, amplifying all 360 degrees of your signal, all the time. Now let’s say we have two lumberjacks, one on each end of the saw. The first lumber jack pushes the saw through the initial half of its journey or let’s say 180 degrees of rotation in its cycle. The second lumberjack then takes over and pushes the saw back through the log completing the second half of the saw’s total cycle. In this example the saw has traveled the same distance with only half the effort from our original lumberjack.
To examine the difference relative to “work done” let’s move to the playground and set our sights on an ordinary children’s seesaw. If our friend Billy is playing alone on the seesaw he can push off the ground driving it upward and through its range of motion, moving his own bodyweight through the seesaw’s 360 degrees of travel. We could say that Billy is operating in single-ended fashion. Now if Billy’s friend Jill hops on the other end of the seesaw we can watch Billy push with the same amount of effort on his half of the seesaw while Jill pushes equally on the opposing half of its cycle. In this example it is clear that Billy is doing the same amount of work he did when he was riding solo, Jill is mirroring his effort, and the seesaw is now moving twice the weight.
Class A describes an output stage in which the power tubes amplify 100% of your signal all the time and do not spend any meaningful time in cut off. To get any appreciable power from pure Class A the tube must be biased as hot as possible (to allow maximum signal swing before cut-off, i.e. zero plate current). This is the least efficient mode of operation. These amplifiers are typically cathode biased but can be grid/fixed bias as well.
Class A/B describes an output stage that is necessarily push-pull and is a hybrid of Class A and Class B. This operating class begins with the tubes operating in Class A and as the amplifier is driven towards full output is pushed into Class B. This arrangement is normally biased much cooler than Class A and is a more efficient mode of operation. It’s also the most common topology used in guitar amps. This arrangement can be grid/fixed bias (typically seen in amplifiers exceeding 40 watts) or cathode biased (generally in amps 30 watts and below).
Class B describes an output stage that is necessarily push-pull and where the tubes start operation in cut off (biased with zero plate current flowing) and are driven upward from that point. This is the most efficient class of operation and will provide the most output of the three classes described. This arrangement is virtually always grid/fixed bias and is seldom seen in tube audio circuits.
A special note about Class A/B push-pull amps that are advertised as Class A. This category would include the classic Vox AC30, Marshall 18 watt, and all their derivatives. These amplifiers follow the normal A/B Push-Pull arrangement but are also cathode biased. With the cathode biased approach the designer can bias the tubes much hotter (i.e. flowing more plate current, typically right at or even over the tubes maximum ratings). The hotter bias setting allows the tubes to spend more time in the Class A region but as the amp is driven towards it’s full power output the tubes will still fall into Class B, so the A/B designation is the most appropriate.