What the heck is the gas burner in my furnace doing in an article about tubes? Is it a misteak? Well, if you look closely at the upper right area to the little white ceramic insulator with a wire attached to it and a corresponding black ‘stick’ protruding into the right hand burner flame, (and if you understand how the flame sensor works – ask Mike FAN) then you might see the connection.
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England’s John Fleming invented the vacuum tube diode ‘Valve’ back before the turn of the century, not this one, the one previous. There were two elements inside the glass tube vacuum, one heated and another with a (large) positive voltage on it, and it proved that ‘Electricity’ was something that could flow from one piece of metal to another Through A Vacuum (i. e. Across space).
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Lee DeForest (Council Bluffs gets to say he was born here – family left when the kid was only 4, but He Was Here!) added a third element to Fleming’s Valve, and lo, you had a real tube, that is, you could control the flow of electrons if the third element in between the other two was charged more or less negative. Amplification!
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More will be added to this article shortly, but for now, what’s the deal with the ‘element’ in the flame above? Is there current flow? Which direction? Hint: Not much current and it’s ‘backward’.
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DeForest’s tube, he called it an Audion – more about that later – was originally a chem lab test tube like the ones you may have used in high school, with a rubber stopper through which he pressed four (4) wires, two for the heated ‘Cathode’ where the electricity comes from (electrons jump off it), a ‘Plate’ like Fleming’s diode tube, and a zig zag piece of wire between, the ‘Grid’ that controls current flow between Cathode and Plate if you put a little varying negative charge on it. Oh, also a piece of glass tubing so you can suck the air out.
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Leo Meyerson WOGFQ had an original DeForest Audion in his boardroom at WRL, 35th and Broadway, in the sixties. Not everyone got to see it. It was one made from a chem lab test tube (upside down – see diagram below). Later Audions and all commercial ones looked more like a light bulb, like the early tubes, a ‘globe’ of glass with things inside and a real metal base with wires coming out.
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DeForest somehow got the idea to put low pressure gas inside that would become ionized. Thus the suffix on the name Audion. Early models had varying amounts of gas. The gas ruined the tube characteristics. Some worked well, and others not so much. It took a couple years for someone to suggest a perfect vacuum. That’s when tubes started getting useful.
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So back to the gas furnace with the metal stick on the right. It’s a flame sensor. If it doesn’t sense flame, the furnace turns the gas off. They apply a small 6 volts of AC to it, and when the flame turns on, current flows from stick to ground (furnace chassis) through the ionized gas flame. Not much, microamps vs millis, but enough with modern electronics to say yes, the flame is on. Backwards from a tube. Negative part of AC on the cathode (stick) through a slightly conductive ion path, to plate (furnace chassis) and sense something like 2 to 10 uA (microAmps) of current. Like it?
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