Explanation: Class A vacuum tube amplifiers are designed for maximum linearity and fidelity, with the tube conducting for the full 360-degree cycle of the input signal. To achieve this distortion-free operation, a significant "quiescent current" must flow through the tube even when no input signal is present. This continuous current draw means the amplifier is constantly dissipating power as heat. A large portion of the DC power supply energy is thus wasted, limiting its efficiency. While the theoretical maximum efficiency for an ideal Class A amplifier is 50%, practical circuits typically achieve only 20%-30%. Higher efficiencies, such as 60% (Class B/AB) or 85% (Class C/D), are attained by reducing the conduction angle or employing switching techniques, often trading some linearity for improved power conversion. 100% efficiency is unattainable due to inherent losses in any real-world electronic circuit.