Class A Amps

There is a significant amount of marketing hype and misinformation out there pertaining to "class A" tube amplifiers. Somewhere along the way, the term "class A" became a marketing buzzword to sell tube amplifiers that were purported to have highly sought-after classic tonal qualities. The truth about the tonal qualities of an amp is much more complex than just the class of operation of the amp.
This is a fairly technical discussion, so first, a few definitions:
Saturation: The maximum point in the swing of the waveform, which typically produces the highest amount of current and lowest amount of voltage. The valve (tube) is said to be "wide open" or "fully on" at this point.
Cutoff: The point in the swing of the waveform where the voltage is highest and current is 0. The waveform is no longer amplified and the valve (tube) is said to be "off" at this point, hence the name "cutoff."
Class A: A class A amplifier is one that is biased in a way that allows the plate current to flow for the full, entire 360 degrees of the input cycle. Therefore, the tube is always "on," current is always flowing and it never goes into cutoff. Due to the inefficient nature of this class of operation, class A tube amplifiers tend to be lower powered amps.
Class AB: A class AB amplifier is one that is biased in a way that allows the plate current to flow for more than half of the full 360 degrees of the input cycle, but much less than the full 360 degrees of the input cycle. The tube is allowed to go into cutoff and "rest" for a short period of time. This class of operation is more efficient and is a popular class of operation for medium-to-high powered tube amplifiers.
Push-Pull: A push-pull amplifier utilizes opposite sets of power tubes - usually a pair or quartet - to conduct the alternate cycles of the input waveform. If more than two tubes are used, it is called "parallel push-pull," as the additional tubes are configured in parallel on each side of the output section. The input waveform is typically "split" by the phase splitter before the power tubes and then each half is passed to the input grids of each set of power tubes. It requires a center-tapped output transformer, where the two amplified halves of the waveform are then combined back together as a single waveform before driving the speaker. Push-pull amps allow for the use of smaller transformers and have the added benefit of power supply hum cancellation. They also minimize harmonics and distortion. It is a complex topology, but one that is typically more efficient, generates more power output and minimizes crossover distortion. Push-pull amps can be biased for class A operation, but are typically biased for class AB operation.
Single-Ended: A single-ended amplifier is a simple topology where the entire input waveform remains intact and is completely amplified by one or more power tubes. Because of this, there is no need for a phase splitter in a single-ended amp. Multiple power tubes can be tied together in parallel, creating what is called "parallel single-ended." Also, the output transformer is not center-tapped and has a special "gapped" construction to prevent core saturation. Single-ended amps require larger transformers and do not reject the power supply hum like the push-pull configuration does. However, they do emphasize even-ordered harmonics and asymmetrical clipping which are considered more pleasing. This topology is less efficient and commonly utilized in lower powered amps. Single-ended amps are typically biased for class A operation.
Cathode-biased: The bias method of the power section of an amp where it is self-biased via a cathode resistor and capacitor. Typically with this method, the tubes runs "hotter" and the output is less efficient.
Fixed-biased: The bias method of the power section of an amp where it is biased via a negative DC voltage applied to the grid. Typically with this method, the tubes runs "cooler" and the output is more efficient.
Negative Feedback: A method of injecting a small amount of signal from the output transformer back into the phase splitter to flatten and extend the frequency response. It makes the amp's output stage cleaner, reducing unwanted distortion and improving stability. Some amps are designed without negative feedback which allows the output stage to produce more harmonics and have a more dynamic response.
There are a few schools of thought when defining an amp and its class of operation.
A strict purist might define the "true class A sound" as a cathode-biased, single-ended class A amp with no negative feedback. There are advantages and disadvantages to this topology, but sonically, there is a pleasing harmonic overdrive, saturation and response to this amp configuration that many guitar players love. This amp topology is often what people are referring to when they yearn for the class A sound.
Those that are looser with the definitions would possibly call a cathode-biased, push-pull amp with no negative feedback a "class A" amp, like the circuit in the SonicTone Royal Crown amp line. That type of amp circuit and many of its iterations have famously been marketed as "class A" amps over the years. However, it could be argued that these amps are not class A, but simply cathode-biased, push-pull, class AB amplifiers that have no negative feedback. But, that particular topology does give an amp a very "class A-ish" sound.
In fact, there have been amps marketed to have a switch that claims to switch the amp's class of operation from class AB to class A. In actuality, all the switch is doing is switching the bias of the amp from fixed-biased to cathode-biased to give it a more "class A-ish" sound, but the amp is still technically operating as a class AB amplifier.
The amp models in the SonicTone SE Series are the only pure, single-ended class A amps in the SonicTone line-up. The rest of the SonicTone amplifier line-up has a number of cathode-biased and fixed-biased push-pull amps, and they are labeled accordingly.
In summary, here are some helpful facts:
- A class A amp can be either single-ended or push-pull.
- A class A amp can be fixed-biased or cathode-biased.
- A class AB amp can be fixed-biased or cathode-biased.
- An amp being cathode-biased is not a definitive indicator of class A operation.
For more information on the class A topology, please see these reference links: