Understanding the Valve. by Phil Moss, Curator of Radio

Understanding the valve. By Phil Moss, Curator of Radio The Valve: those enigmatic things that glow down the middle (subject to not being covered in gold paint, for example), which probably seem mysterious but are the basis for all original electronics. There was no electronics before the invention of the valve. The thing is, they are not that difficult to understand, and even to build things with. This is an introductory article and does not intend to cover the fine details, but only the general principles. It will enable you to understand the outline of radio and audio applications without having to “Strain the Brain” much. In the section under the authorship of Keith Wevill there is a longer article: Valves and Valve Testing, which among other things contains rather nice illustrations, and you may wish to read that after this but it was though written with an audience in mind who are already somewhat familiar with the subject. I have made reference to some uses without explanation here: but they are to be found in my other article on Methods of Reception. So, In The Beginning, let there be Light. Light bulbs in this case, because it was work on the inadequacy of light bulbs which led to the development of the valve, and also it was the light-bulb industry that in general also was to make the valves. The problem of bulbs was that when they had learnt to make them last reasonably, in terms of not blowing the filament, but they became dull because of the tungsten filament slowly boiling off and the metal depositing on the glass. Research into ways of stopping that eventually led to adding a very low pressure of chlorine gas in an otherwise hard vacuum, the chlorine molecules managing to bounce the tungsten atoms back onto the filament. But that came later and is not part of our story. Before that other things were tried. One of which was to try to attract the gaseous tungsten onto a metal plate. An experiment was done to see if applying a voltage to that plate would cause the atoms to go to it. Therein lay the great discovery: that when the plate was positive with respect to the filament a small current would flow, but when it was negative it wouldn’t. Whether it worked in attracting the tungsten seems not to have been recorded for history, but no light bulbs were made commercially with this plate added as far as I know. This conduction it should be noted had little or nothing to do with attracting tungsten atoms, it was attracting electrons, when the anode was positive. This is critical: valves work by the flow of electrons emitted or boiled-off the filament. The great significance of this discovery was that a device was now available that conducted one way. In fairness it was not the first: the famous “Cat’s Whisker” detector had long pre-dated it, and before that devices such as the coherer which detected radio waves but were only useful in receiving Morse, and did not work by rectifying action. Now there was a reliable detector of radio waves, which didn’t need fiddling with, albeit that it did need a battery of considerable current capacity. The real significance however was where this would lead to. The diode as this valve was called: because it had two electrodes: filament and plate, or as we more often say in the UK, anode, was invented by John Ambrose Fleming (relative of Prof. Sir Alexander Fleming of penicillin fame). However as with most things, one finds others had been doing similar things. He was a professor at University College London The diode in itself did not change the world. However some time later Dr Lee de Forrest in America had a bright idea: he added a mesh of wire between the filament and anode. He demonstrated that the voltage on this could be used to control the anode current. This made the valve an amplifier, and that would in time change the world profoundly. This was not strictly the first amplifier: a device that could give more output than input, there were magnetic amplifiers, but of very limited capabilities. The valve would prove exceedingly versatile. Before I go further, an explanation may be needed: I have referred above to more out than in, which superficially infers that the Laws of Nature are breached by the valve: something for nothing. This is not the case: the power for the greater output is provided externally, initially from batteries, later the mains. The First Law of Thermodynamics is not breached. Lee de Forrest’s triode (triode: tri = three: so three electrodes) didn’t change the world either any time soon. This was because he didn’t really know how to use it. That took quite a time: what appears obvious to us was not to him learning for the first time. Patent disputes took up his time too! There was also a fundamental misunderstanding on his part: he believed that gaseous ions were part of the process: they are not, what one needs is the hardest of vacuums, and there lay another problem: although there were vacuum pumps for light-bulbs, they were not as good as would be needed in valve production. They were good enough for the original valves and with time better ones were devised, and also a method of getting the last traces of gas that remained, logically enough called a Getter. So let us now move on to a time when a reasonable valve had been made. Expensive, and power- hungry for the filament, which ran like light-bulbs close to the melting-point of tungsten, at 2400 degrees C. They were known as bright-emitter valves as they were as bright as small light-bulbs, and had much the same life: not that long. However, for those with deep pockets, radios that did not have a confounded “Cat’s Whisker” detector to fiddle with and were a lot more sensitive were now available. And the growing popularity of radio meant that it spread quickly, even to those who had to save hard to afford it. Even crystal sets were expensive. The Military were big users of radio: the First World War pushing technology forward faster, as did the Second would. Valve production increased and methods of automating some processes were invented. Also how to use the triode as a radio detector and amplifier better were evolved by Marconi: though he charged a great deal for use of the patents. The result though was far greater amplification and selectivity. The high-frequency characteristics were not good. This is due to the interaction of anode and grid. I am keeping this simple so I will just say capacitance and Miller effect. You can look these up elsewhere. A new development was needed. Enter the Screened Grid Valve, later known as the tetrode (Tet = four). The new grid was between the control grid: that is the one we have met, and is there to control the flow of electrons, and the anode. It screened the two so that the control grid couldn’t “see” the anode. Capacitance between anode and control grid was now reduced to vanishingly low values. The new grid was held at a steady positive voltage, and at AC, that is the signal frequency was decoupled so there was no signal voltage on it. The result was a valve with very much higher gain (gain = amplification) and that would work at very much higher frequency. They tended to be used as the first stage in a multi-valve set, though they would make a very good valve in a single valve set, but I am not aware this was ever done, at least commercially. This worked fine provided that the anode voltage was always kept higher than the screen. That was almost always true in signal amplifiers, but when used as an output valve to drive a speaker, the anode voltage would often swing below the screen. The result was that the anode current would drop, and the screen current would increase, greatly. The reason was that electrons are attracted to the most positive point, but also there was another cause: electrons hitting the anode hard were able to knock electrons out of the atoms of the metal used to make the anode. These are known as secondary electrons. These would naturally go to the screen grid as it was more positive. Philips cured this by adding another grid: the Suppressor, which was connected to the filament, and was therefore much more negative than the anode or screen: it repelled the secondary electrons back to the anode, whilst having little effect on the fast electrons from the filament that had been accelerated towards the anode by the screen grid. This new valve with five electrodes was therefore called the Pentode. Pentodes became very popular for both high and audio frequency use and at power and signal levels. Other very important developments were firstly the use of thoriated tungsten filaments. Adding the element thorium greatly increased the amount of electrons emitted by the filament, such that the temperature could be dropped to a “mere” 1700 C. This greatly reduced filament current and greatly extended life. There were other better solutions to increasing emission, but not always very practical from the manufacturing point of view. Finally the Rare Earth Oxide type was developed: using rare metal oxides on the filament which enabled the temperature to be dropped again, now to 850 C.

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