Friday, May 5, 2017

Light Bulb Vacuum Tube

The electronic vacuum tube, once the mainstay of all electronics equipment then replaced by transistors and microprocessors, has a curious legacy. Who is mostly responsible for its development? Fleming, DeForest, Edison, someone else? My vote is Howard Armstrong – here’s why.

Coining the phrase ‘believe it or not’, the vacuum tube is strongly related to the light bulb. Several individuals conducted light bulb research with Thomas Edison ending on up top of the heap in 1878. Edison’s light bulb was the first discovery in vacuum tube evolution. In 1875, while testing various voltage levels on light bulb filaments, Edison noticed that at some point too much voltage to the filament would eventually burn it then disintegrate it. When this happened, it left a black carbon film on the inside of the tube. He also noticed that there was a slight clear line on the glass opposite where the filament once stood. He then theorized that the carbon carried an electrical charge; that is, electrons flowing throughout the glass bulb but he couldn't explain why. This phenomenon became known as the “Edison effect” [thermionic emission for you old tube guys] and is classified as an electronic tube [flow of electron between electrodes]. 
Fleming's Tube

Professor John Ambrose Fleming of England [was also a consultant to Edison in the 1880’s] invented the vacuum-tube rectifier in 1904. The Fleming Valve, as it became to be known, was a two-element device inside an incandescent lamp. The elements were made up of a filament [usually carbon] and a plate. The plate was a cylinder that was wrapped around the filament, and then a lead from each element is exited outside the tube by wire for electrical connection. 

Operationally, it was the first diode and is considered the genesis of electronics. It was broadly used in radio designs as it converts AC to DC, needed to detect electromagnetic waves. Fleming’s Valve replaced the ‘cat whisker’ that was used to detect radio signals in receivers during this time.

It is noteworthy to comment that Fleming also worked for Marconi and designed the transmitter that was used for the first radio transmission across the Atlantic in 1901 [Puldhu, England to Nova Scotia].

Next in line is Lee DeForest, who experimented with Fleming’s valve, and is credited of inventing the vacuum tube triode – the audion in 1906. The DeForest audion now could amplify AC signals, going beyond the switching valve converting AC to DC.

DeForest Audion
He altered Fleming’s design by adding a third element called the “grid”. The grid was a wire mesh that was placed between the filament and the plate. By applying a small current to the grid, there would be a corresponding larger current flowing in the plate – amplification. The audion became the main component of radios for the next several years, widely improving reception. For this, De Forest is known as the “Father of Radio”. 

Deforest is considered a impostor by many engineers as he just improved upon Fleming’s design and never understood how his audion really worked. Although it was invented in 1906, it wasn't until about 1912 -1913 that it began to be used because no one really knew how this device worked. While the audion was not entirely reliable and was plagued with problems that prevented widespread use, it was state of the art at the time. DeForest sold the audio patents to AT&T who used the audion to amplify voice on their wireline circuits.

Now, the vacuum tube, Edison started it but couldn’t explain it, thereby shelving it for future research; then Fleming finding use for it and DeForest who expanded its capabilities.

Enter Howard Armstrong – genuine engineer and gifted inventor. In 1914, he worked with the audion and did what DeForest should have done - he analyzed it! By applying varying voltages, signal levels, taking and recording measurements – essentially producing what are called vacuum tube characteristic curves that shows how the tube operates under certain voltage/current conditions. All tubes and transistors have characteristic curves to allow engineers to design specific types of circuits [such as amplifiers, oscillators] based on these curves.

Armstrong experimented with the audion, trying different tests and measuring their results, while observing their effects based on experimentation. Armstrong knew that by feeding a signal at the tube’s input, there was an amplified output. One of Armstrong’s experiments was to take some of the output signal and feed it back to the input circuit – what he found was that the amplifier was able to significantly amplify weak signals to greater levels. This is what radio needed at the time, a device that could amplify very weak signals. This was his first invention, it became to be known as the “regenerative circuit”, and a few years later invented the superegenerative circuit that lead the radio revolution.

Furthermore, his genius didn't stop there as Armstrong also found that by applying too much signal back to the input to maximize amplification sent the tube into oscillations - an unstable and undesirable condition for receiving signals. Not to Armstrong, he found that his regenerative circuit, if set up correctly, ended being a reliable transmitter of signals. Imagine that, a greatly improve tube circuit for receiving very weak radio signals and greatly amplify them and a new transmitter design of radio signal all at one time – simultaneously!

The signal transmitter was a significant finding, for at the time continuous wave transmitters were large motors [alternators] the size of automobiles – a simple vacuum tube was now replacing this technology!