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ALL ABOUT VALVES   

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PILOT  BUSH  EKCO  COSSOR  PHILCO  BAKELITE HISTORY VALVES DIODE, TRIODE  TETRODE, PENTODE  STANDARDS     

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.  

From the start the main difficulty was to seal the connections to the internal electrodes where they came through the glass envelope. Many variations were tried, using a plastics base to the valve, bonded to the glass and acting as a support for the lead out wires which were terminated in strong metal pins. The British 4, 5 and 7 pin series are examples. Side contacts were tried for a while. These and other later types that were all-glass with a 'pip' on one side 'snapped' into spring-tensioned valve  holders (sockets). It was widely thought that valves would work loose if they were not retained in some way. This may well have been true for WD and communications equipment, but for most domestic radios, especially those with standard horizontal chassis, the problem simply didn't exist. The later miniature valves -B7G and B9A - needed no such fixing as the holders gripped the pins quite firmly, though whenever they were used horizontally as in many TV sets, there would be a wire clip for security.  

 

Similarly with screening. Lots of early valves designed for use at RF were 'metallised' either with a grey or a gold conductive coating. The earlier grey in particular tended to flake off. As this screening was part of the structure of the valve, carried by a fine wire to earth (chassis) through a convenient pin, these valves can cause problems such as instability in operation. The later and physically smaller valves dispensed with screening and opted for external metal cans that had a simple  bayonet fixing into sockets on the holders.

The early valves had direct filaments and were designed to run from DC supplies, which meant in practice batteries or accumulators. The development of the separate heater allowed sets to be designed for AC mains.

Battery valve design, basically for portable or transportable sets, improved over the years. From the 2-volt filament types came the so-called 'all-dry' range of 1.4 volt filament valves. All-dry meant that no accumulator was needed for the filaments. The earliest of these were side-contact-based but these were quickly supplanted by octal style valves, usually in a tubular form with a plastic base and very elegant they were, too. I have a soft spot for them because it was this type of valve, specifically 1N5/DF33, 3Q5/DL35 among others I used when building my first radios, the 1952 Practical Wireless modern 1-valver, 2-valver etc. Because they used less current than the older 2 volt types they were used in both portables and small table models for those without mains supply, until supplanted by miniature B7G all-glass types, which retained the 1.4 volt filaments.

Basic types of valve

Diode: two active elements, anode and cathode (or filament in battery valves)

Triode: three active elements, anode, grid and cathode

Tetrode: four active elements

Pentode: five active elements

Some tetrodes and pentodes may be termed 'variable-mu'. This means that their gain can be controlled more easily by externally applied voltages, making them useful in automatic gain designs. They are found in RF amplifying stages.

There are also many variations of double valve, the common ones being triode-pentode, double triode, double diode triode, triode hexode, triode heptode. The latter two are used as frequency changers/local oscillators in superhet designs. Their name tells you they have even more elements!

 

 

 

ABOVE: an  'R' valve, c.1917

ABOVE: a typical mid-thirties Mazda valve

A typical all-glass radio valve from the late 1950s - early 1960s

ABOVE: a B9A all-glass miniature valve, common in the later years of the valve era

 

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