what is a class A amplifier?
A class A amplifier is defined as one which is biased to a point where plate current in all the output devices flows for the entire 360 degrees of an input cycle, at the full, unclipped output of the amplifier. This is typically done by biasing the output stage halfway between cutoff and saturation, with the plate load impedance to an appropriate value that gives maximum undistorted output power. This is the least efficient method of amplification, because the output devices are dissipating maximum power with no input signal.
For audio amplification, a class A amplifier can be either single-ended or push-pull. Now, you might be thinking, how can a push-pull amplifier be class A? Doesn't one side amplify half the waveform and the other side amplify the other half? Isn't this why we use a phase splitter? These are common misconceptions. You can, indeed have a true class A amplifier that operates in push-pull mode. Amplifier class has absolutely nothing to do with output stage topology. If the output tubes on either side of a push-pull pair are biased in class A (halfway between cutoff and saturation), then the current in each side will still flow for the full 360 degrees of the input cycle, just in opposing directions. As one tube's current increases from the midpoint, or idle, bias current, the other tube's current is decreasing by an equal amount. The output transformer sums these oppositely-phased currents to produce the output waveform in the secondary winding. As one side reaches saturation, the other side reaches cutoff, just as they would in a single-ended class A amplifier. Neither side cuts off at the full, unclipped output power of the amplifier. The output power of a push-pull class A amplifier is exactly twice the output power of a single-ended class A amplifier operating under the same conditions of plate voltage, bias, and effective load impedance.
Another misconception is that of cathode biasing. The method of biasing has nothing to do with the class of operation. You can have a fixed-bias class A amplifier or a cathode-biased class AB amplifier, or vice-versa. The presence of a cathode bias resistor and bypass capacitor is not an indication of class A operation.
There are several advantages to push-pull class A amplification. First, the bias current for each side is flowing in opposite directions in the primary of the output transformer, so they effectively cancel each other out. This lack of static, DC offset current in the output transformer means that the core can be made smaller, because it requires no air gap to prevent core saturation from the static DC offset current. A single-ended class A amplifier output transformer is huge compared to a push-pull class A amplifier of the same power level. The air gap required to prevent core saturation drastically reduces the primary inductance, so the transformer must have a larger core and more windings to achieve the same primary inductance and the same -3dB lower frequency cutoff point. Second, a push-pull class A amplifier output stage will have inherent rejection of power supply ripple and noise. This is because the power supply signal is "common-mode", i.e., it is amplified by each side equally, but since each side is out of phase, it cancels in the output.
The main disadvantage of push-pull class A amplification over single-ended class A, is the necessity for a phase splitter stage to generate the oppositely-phased drive signals. Another "disadvantage", in terms of guitar amplification, is that even-order harmonics generated in the output stage are canceled out in a push-pull output stage (hi-fi guys consider this a great advantage, by the way!). This does not mean that the amplifier generates no even order harmonics, however, because even-order harmonics generated in the preamp stages will pass right through to the output. Only those generated in the output stage itself are canceled.
