Pure A class is the most simple topology for designing an amplifier! The tube or the power transistor is loaded directly by the speaker (often thru a transformer, fro impedance matching!), and is biaised at the current necessary for full modulation (that means a maximum power dissipation for no input signal).
The consumed to output power ratio is very low: 12.5% for a current source load (see PASS amplifiers), 25% for a transformer loaded triode with no grid current, 50% for the same triode in class A2 (with grid current) and 50% for a pentode (or tetrode). Un