Preamp valves are usually triodes, with pentodes occasionally used and tetrodes not featuring at all. In contrast, power triodes are rarely used as power valves, but tetrodes and pentodes are commonplace. That said, power triodes have a following in hi-fi circles where their supposedly superior audio characteristics take precedence over sheer volume.
Triodes had been around since 1906 and tetrodes can be traced back to Walter Schottky’s work on bi-grid valves in 1919. Early tetrodes were devised to outperform the high-frequency response of triodes, but they were only partially successful, for reasons we’ll discuss later. Meanwhile in Holland, valve technology was about to take a big step forward.
Pentode Push
Bernard DH Tellegen and Gilles Holst invented the pentode valve in 1926, while they were working for the Philips Physics Laboratory in Eindhoven. Mullard was a subsidiary of Phillips and the name would later become synonymous with the best pentode power valves.
Power and preamp pentodes work in exactly the same way, as we described in last issue’s Nitty Gritty column. With
