File:Marconi Carnarvon transmitter valves.jpg

Since the power output of early valves was limited, many were used in parallel. The transmitter used 48 (later increased to 56) 4 kW Marconi MT1 valves, developed by Marconi engineer H. L. Round. The valves were 14 in long, the size of footballs, used a plate voltage of 12,000 V and had a lifetime of around 5000 hours. They are constructed so it is easy to replace the filament when it burns out without damaging the tube. Each had its own grid leak capacitor. Without central excitation, the device was unstable and difficult to adjust compared to modern transmitters. Along with a similar transmitter in Glace Bay, Newfoundland, this transmitter was used for commercial radiotelegraph traffic between the US and UK. In December 1921 it made a historic direct radiotelegraph transmission to Australia, a distance of 12,500 miles, at an input power of 160 kW.