Above, left: BTH 'R' valve, circa 1918.  Picture from 'Introduction to Valves' by Hallows and Millward, Iliffe, London, 1953. .Above, right: B9A all-glass valve, a form popular with manufacturers during the later stages of the valve era

It's quite possible that you've never encountered valve technology, especially if you were born at any time from the late 1960's on. It is, for sure, many years since valve radio manufacture ceased.

NOTE for American readers: the terms 'valve' and 'tube' are interchangeable in this document. The latter name is used in the USA and is simply an alternative to valve. Similarly , 'plate' and 'anode' are alternate names for the same electrode.

Terminology has changed over the years, too. The letters 'HF' stood for 'High Frequency' which is now usually termed 'RF' - Radio Frequency. Therefore 'HF amplifier' and 'RF amplifier' are also interchangeable.

James Clerk Maxwell, a brilliant Scottish physicist, first developed a theory about electromagnetic waves in 1864. He based this on even earlier speculative work by other scientists. He believed that electromagnetic (i.e. radio) waves would behave like light. If light could travel through space, so could electromagnetic waves. This theorising was laboratory-proven in 1888 by Heinrich Hertz, another brilliant physicist, German by birth. Unfortunately he did not live to carry out advanced practical testing of his work as his lifespan was even shorter than Maxwells. 

It was left to Marconi to first demonstrate a form of wireless telegraphy. It is often stated that Marconi invented radio. Though basically true, this is a simplification and the reality of radio's beginnings is much more complex. In fact, the British experimenter Hughes had demonstrated radio transmission some years before Marconi - but it seems the scientific community did not believe him! 

Early radio could only use morse code. The transmission of speech and music required the development of the amplifying valve and continuous wave transmission system, where an oscillator generates a carrier wave of high frequency which is then modulated by another wave, an electric analogue of the original sound. At the receiver, the high frequency wave is removed and the speech or music analogue is amplified and fed to a loudspeaker to change it back into sound.  

The valve, therefore, was crucial to the development of radio. The crystal set, which used a crystal 'signal detector' to detect (demodulate) the sound from the carrier, could not amplify. These devices could normally only be used by one person at a time, wearing headphones, and as they were powered by the strength of the received signal, were, for reliable results, limited to a distance of around fifty miles radius from the transmitters of the time. It is interesting to note that modern electronics is based upon solid-state, i.e., crystal, semiconductors!  

John Ambrose Fleming, an English physicist and electrical engineer, invented the diode valve in 1904. This device contained a filament similar to that of a light bulb plus an 'anode', a metal plate carrying a high voltage charge. Fleming found that electric current would flow from the filament to the anode, but not from the anode to the filament, hence the name 'valve', from 'one-way valve'. This device could be used instead of a crystal, but amplification by the diode valve was only possible after Lee DeForest, an American physicist, added a third electrode in the form of a spiral of fine wire called a grid, fitted between the two electrodes. Small signal voltage changes at the grid could be made to result in large voltage changes at the anode. DeForest patented his triode (three-electrode) valve in 1906. The amplifying valve was born, and with it, radio for the masses.  

Left: a typical mid-thirties Mazda valve