It has been said that it was after the advent of steam that the greatest progress was made in the design of sailing ships. Be that as it may, it is interesting to note that at the 1982 International Broadcasting Convention at least two of the most interesting new developments were in the form of thermionic devices. Thomson-CSF described their new TH539 tetrode for MF broadcasting that is capable of delivering no less than one megawatt of carrier power. And if you still think in terms of receiving-type valves it may be a shock to lean that the TH539 has a heater taking 30 V at 900 A and an anode current of 90 A at 13 kV
But a more fundamentally radical development is the new single-cavity UHF 'klystrode' developed by Varian. This, as the name implies, is half klystron and half tetrode and offers the possibility of developing a high-power, Class B, linear amplifier for vision or sound transmissions, as a means of improving overall conversion efficiency. Early power klystrons in vision service were only about 25 per cent efficient at peak sync. output. Since collector current remains unchanged regardless of the input waveform the true conversion efficiency, in terms of kilowatts consumed from the supply mains related to output, was only a few percent. Since then considerably higher efficiencies have been achieved both in the basic klystron design and by the use of pulsers to reduce collector current at other than sync. periods (EEV devices tested on the BBC Crystal Palace transmitters have achieved 70 per cent, measured on the basis of peak sync. output) but the klystrode appears to be the first klystron-type device to offer the possibility of Class B operation. But already, for MF, all-solid-state sound radio transmitters are now available up to 10 kW output.