Some of the early transistor designs were very poor by modern standards, but some compared favourably against 'equivalent' valve circuitry. By the 1970s, most development of valve amplifiers had ceased, since the 'writing was on the wall'. With the advent of the transistor in 1948 there was a whole new design process to master once transistors became commercially available. 'True' tetrodes had a limited production, because they didn't work very well (hence the pentode). Early valve amplifiers used triodes, because they had nothing else until the pentode was invented in 1930, with the beam-tetrode following not far behind (to avoid the patents held by Philips). It was quickly discovered that push-pull was superior to single-ended operation in all respects.
Once people realised that it was possible to amplify weak signals to drive a loudspeaker (especially for wireless, public address and 'talking' movies from 1927 onwards), the race was on to get more power. Initially, valves were used primarily for radio/ 'wireless' detection and amplification, along with telecommunications (the latter has been a primary 'driver' of electronics development until fairly recently). Push-pull operation could increase output by a factor of at least four. Most of these early single-ended amps would struggle to get even 5W output, using large power triodes. Single-ended triode amplifiers were initially the only option, until design skills improved and new designs were developed. Early amps were very low power, and were paired with highly sensitive speakers, often using horn loading to get more 'noise' with the limited power available. The primary goals were usually to get more power with less distortion, and this quest continues. Since the very first power amplifiers were developed in the 1920s, there have been countless different designs. 11 A (Relatively) Modern Discrete Power Amp.
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