Triode Vacuum Tube: How it Was Invented & How it Works

Before the invention of the transistor the most important electronic equipment was the vacuum triode.  All radios were driven by them, televisions used them, heck, even early computers had rooms full of these jacked up light bulbs.  But how do they work, and how (and why) were they invented?  This is the story of a smudge in a light bulb, an assistant with a good memory, and a con man working around a patent.  Ready?  Let’s go!

It all started in 1880 with the regular light bulb. Thomas Edison had noticed that, when his light bulbs broke, they would create black smudges on the inside of the bulb near the positive end of the filament.  He had worked very hard to make them good vacuums, so he was pretty sure the black marks were not made from gas inside the bulb, therefore, something must have come from the filament itself.  He then added a plate to the bulb, with the hope that whatever was emanating from the filament would stick on the plate and possibly keep the filament from breaking in the first place.  The plate turned out to be not much help, however, he did notice that if he added another battery between the filament and the plate, something strange happened: the current could flow if the plate was positive, but would not flow if the plate was negative!  Edison patented it as a voltmeter, although he didn’t really know what was going on or why anyone would want this expensive and ineffective voltmeter.  What was going on?  It all has to do with the motion of negatively charged electrons.  In the light bulb when the filament got hot the electrons could become free.  When the filament broke the electrons would fly towards the positive end of the filament.  If, before the filament broke, you added an extra voltage between the filament and a plate and the filament was negatively charged and the plate positive, the negatively charged electrons could easily “jump” off of the filament and onto the positively charged plate and let the current flow.  If, however, if the plate was negatively charged, the electrons would stay on the plate as it was cold and not concentrated on a thin filament.  This was called the “Edison Effect” although aside from patenting it Edison wasn’t particularly interested in it!  However, Edison had a young English technical advisor named Ambrose Fleming who was interested.  Fleming was there to help Edison with alternating current which Edison quickly decided to reject and Fleming went back to England.

Jump forward 24 years to October of 1904.   By this time Fleming was a technical advisor to Guillermo Marconi and was helping him with sending wireless telegraph signals.  Marconi’s biggest problem in 1904 was in his receiver.  He used a tube with little filaments that would stick together (or cohere) when a radio wave went through it and then would be tapped when the signal was complete.  It was slow and awkward, According to an engineer at the time, the coherer, “was publicized as wonderful, and it was wonderfully erratic and bad.  It would not work when it should, and it worked overtime when it should not.” 

The year before, in 1903, a Canadian named Reginald Fessenden had made an “electrolytic detector” that used a chemical reaction to rectify or filter out one direction of the current so that it could be heard on headphones.  Fleming found that it did not work at high frequencies (and they didn’t hold the patent).  He was thinking about it when he had, “a sudden very happy thought… ‘Why not try the lamps?’”  He wrote Marconi, “I have found a method of rectifying electrical oscillations – that is making the flow of electricity all in the same direction.”  By November, Fleming filed for a patent for what was called a “Fleming” valve, or a vacuum diode (“di” for two, “ode” for path)

Note that the “Fleming” valve was exactly the same as the “Edison Effect” bulb, what was different was how he used it.  So, let’s go into a little detail in what Fleming did.  He used an antenna with a coil to receive the radio signal.  He then had another coil so that the alternating current in the first coil would induce an alternating current in the second coil.  He then had that signal go through a sensitive current meter (called a mirror galvanometer) and had one end of the signal go to the plate and one to the filament of his valve (that was heated by a battery).  The valve made the signal one-way so that the meter turned in one direction and could be recorded.  Fleming did not use headphones, possibly because he was partially deaf!  This method worked pretty well, however, it wasn’t particularly popular as it was expensive and most people started using semi-conductors (in what were called crystal sets) as rectifiers instead. 

However, one person was very interested in the Fleming valve and his name was Lee de Forest.  Lee de Forest was being sued by Reginald Fessenden for copying his electrolytic detector, so de Forest was searching for a new detector.  We know de Forest read about the Fleming valve because in December of 1905, de Forest filed for a patent that used what he describes as “an electric valve which has been fully described by J.A. Fleming”.  Then, five weeks later, de Forest patented “a new receiver for wireless telegraph systems” that was strikingly similar to the Fleming valve.  They both had a filament and a plate in a vacuum tube and heated the filament with a battery.  They also took a radio wave from an antenna and passed it through the valve between the plate and the filament.  In fact, there were only four differences.  First, de Forest added headphones instead of a current meter, second he didn’t use parallel coils, third, he added an extra battery (that didn’t seem to do much) and forth he (incorrectly) thought that the trace amounts of gas in the bulb were ionized and what were causing the current flow so he insisted the bulb would NOT be a perfect vacuum.  Because it was used to make sound and he thought it had to do with ionization of gas he dubbed this an “Audion”

Not surprisingly, Fleming sued.  Quickly, de Forest started adding wires and metal all over the place, wrapping his valve in tin foil, or circling the outside or having multiple (connected) plates inside the bulb.  Then, on Christmas day, 1906, de Forest ordered a bulb made with three connections: the filament, the plate, and between them a wire that was bent into a zig-zag shape that he called a “grid”.  De Forest also called this device an Audion although it eventually was called a vacuum triode or even just a vacuum tube.  Adding the third wire in the middle was described in 1922 as the, “the most important single step taken in the whole development of radio communication.”

So what did this wire do? Well, imagine that you setup a vacuum triode by heating the filament with a battery and attaching a separate voltage so that the filament is negative and the plate is positive.  In this case, the electrons would “jump” off the heated filament, past the wire and onto the plate.  Now, imagine adding a weak signal to the grid wire.  If you add a signal where the grid wire had a positive charge on it, many more electrons would be attracted to the grid and jump off the filament and onto the plate.  This would vastly increase the current out of the plate.  If you add a signal to the grid wire with a negative charge, however, it would block the negative electrons from jumping on to the plate, drastically cutting the current.   In this way, small changes in the grid wire could make big changes in the current coming out of the plate.  This was both an electric amplifier AND an electric rectifier (one way valve). 

Unfortunately, Lee de Forest didn’t get his device to work that well and he mostly just used it as a complicated rectifier.  In addition, he was convinced that their needed to be a gas in the bulb (possibly to distinguish it from Fleming’s device) which often resulted in low quality triodes.   Since the “Audion” had variable quality and it was expensive and complicated to boot very few people used them.  It wasn’t until five years later when a tenacious undergrad named Howard Armstrong figured out how to make the triode “sing” by feeding the signal back in a system he called “regeneration”.   This is what made the vacuum triode the Swiss army knife of electrical devices for a full 50 years (meaning vacuum tubes were used for everything!).  And that story starts next time, on the secret history of electricity.

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