A Short Technical Early History of Vacuum Tubes (Electron Valves)

Dor Hesselgrave

Since the dawn of history, man has struggled to control the sources of energy in his world. With fire for his cooking and personal comfort in the cold, with animal power for transport, with running water for his mills, his canal barges..... right on up through the age of steam, he has mastered the energy source. When electrical energy became available, it too had to be controlled. And control it he did, with simple switches to turn on lights and motors; with rheostats to regulate the speed of trolley-car motors and the brilliance of stage lights. However, these controls were all terribly slow, only as fast as man could move the control knobs. Then along came another one of those brilliant concepts that occur through the ages: Lee DeForest's "audion", a vacuum tube with a third element in it that controlled electron flow, the energy stream used in all electrical circuits!

As time went by, we discovered that electrical circuit control using audions could be much faster than the motion of a human hand; the control was at the speed of electron flight, approaching the speed of light. And this was the tremendous contribution that set forth the electronic revolution. We were no longer bound to the rate at which shafts and gears could operate; to speed controls depending upon flying-ball governors; to computation speeds set by the rate of clacking electromechanical relays; to time-keeping standards set by a gravity-dependent pendulum. We now had the low inertia and the speed of the electron serving us.

When following this history of vacuum tubes, bear in mind that these are controllers of electrical energy. Their practical history virtually ended with the invention of transistors, which in turn grew into integrated circuits. In very few years after that, the microprocessor (a super integrated circuit in its own right) picked up much of the control and regulation duties.

This is the story of that chapter in history which begins with the first vacuum tubes and runs into the transistor era.

1883Edison (US), seeking the cause of blackening of incandescent lamps following his invention, discovered current flow inside an evacuated light bulb. The "Edison effect" is current flow from the negatively-charged lamp filament to a positively-charged separate plate located inside the light bulb. At this period, there was not yet an awareness that electrons could flow through a vacuum between two polarized electrodes. In fact, electrons weren't identified until 1897.
1904Sir John Fleming (England) developed a vacuum diode based on the "Edison effect" that could serve as a detector in a radio receiver. This was the beginning of the vacuum tube dynasty, a lineage that used a negatively-charged filament or heater to liberate electrons from a cathode surface, plus other internal control electrodes interposed on the path to a positively-charged collector or anode surface.
1907Lee DeForest interposed an electrode between the cathode and anode of the Fleming "valve" (which is really quite a good name for the so-called vacuum tube, since it can control electron flow in a manner similar to a hydraulic valve controlling fluid flow). With the application of almost zero electrical energy, this third element, called the "grid", could control the large flow of electrons from the cathode to the anode. The grid electrode of the "audion," as this new device was named, functioned much as a handle on a water valve, controlling a huge flow with very little effort. However, the first audions were highly non uniform; as late as 1915, DeForest himself stated that he could not predict whether the anode current would go up or go down when a signal was impressed!
1912Western Electric Co.(WECo) used a modified DeForest audion to build a telephone amplifier. It led the way to coast to coast communication in 1915.
1912H.D. Arnold and Irving Langmuir (US) in independent research found that the audion when operated with a hard (very good!) vacuum became a stable device. It now was tractable to the extent of being able to operate in tandem, with each audion driving the successive unit for higher gain. O. Wehnelt's (Germany) contribution was the invention of the oxide-coated filament, giving a far more copious and lower power usage electron source.
1912Western Electric Co.(WECo) used a modified DeForest audion to build a telephone amplifier. The result delivered coast to coast communication.
1914-1918 World War I period: The demands of the military initiated a tremendous push to build dependable tubes. WECo produced more than 500,000 tubes, named "VT1" and "VT2", for the US Army.
1914-1918GECo (General Electric) copied the WECo design. They changed the filament voltage and the base, and renamed the tube "VT11." More than 200,000 were delivered to the US Army and Navy.
Ca 1915Langmuir (GECo) and Schottky (Siemens AG - Germany) Each developed the space-charged grid tube (tetrode).
1918WECo made the first 50-watt power tube (dubbed "VT18" by the US Navy). RCA (Radio Corporation of America) built the same tube, calling it "UV203."
1919The "equi-potential" cathode was invented, permitting the use of AC voltage on the filament/heater of a tube without introducing any hum into the processed signal. This invention would have eliminated the need for battery operation of the filaments. However, manufacturing problems and cost prevented its adoption.
1920Two tubes dominated the market: the UV200 "soft" (low-grade degree of vacuum) vacuum tube for detector service and a hard-vacuum UV201 for amplifiers. The tungsten filament, very power-hungry and sensitive to voltage changes (which ruled out the use of AC voltages), had to be operated from husky batteries.
1922The invention of the copper-to-glass seal and the watercooling of the vacuum tube permitted huge increases in power handling. Vacuum-tube equipped transmitters jumped up in power from a 500-watt maximum to 50,000 watts. This fact, coupled with the use of frequency-stable tube oscillators, sounded "taps" for the the only other competitive transmitter technique: the Poulsen CW arc device.
1923Westinghouse develops, and RCA manufactures, the WD-11, a tube with a lowered filament power of 0.25 amp at 1.1 volts, using the Wehnell oxice-coated filament. The UV199 "peanut" tubes followed, offering an even more efficient filament (0.06 amp at 3.3 volts), bringing about a great saving of battery power. This same-style filament was installed in the 201 tube, which was renamed the 201A and given a new lease on life as a result of the more efficient use of filament, or "A" battery power.
1927The "equi-potential" cathode was finally put into service (see 1919 above) in the new type 226 and 227 tubes. This type of cathode, immune to noise and hum, was heated directly from a step-down transformer operating from the AC line. The type 280, a greatly-improved diode, with two separate diode plates and a common cathode, was the first of the high-efficiency tubes to be used as a rectifier for the high-voltage AC from an isolating transformer. This combination of tubes resulted in the first hum-free AC operation -- no batteries.
1929The new type 224 tetrode (four-element) tube became the standard amplifier for both TRF (tuned radio frequency) and superheterodyne sets. This was the first advance in function since de Forest's Audion, built on the findings of the Fleming diode (two-element) valve, introduced the triode (three-element) vacuum tube. The extra element, called a screen grid, isolated the input (grid) circuit from the output (plate, or anode) circuit, resulting in much more stable operation and delivering much more amplification for each stage.
Post-1929From this point on, there were few if any significant basic changes to tubes. However, improvements in radio tubes continued at an ever-increasing rate:
  • The cathode heaters became more efficient, saving energy for AC line operation, and permitting operation from smaller batteries for portable work.
  • The cathode emission of electrons, and the tube transconductance (a measure of its capability as an amplifier) increased so much that the conventional triode tubes became useful well into the microwave frequencies.
  • The size of each tube was reduced, for improved miniaturization.
  • The power capabilities increased. Small tubes could now drive big TV picture tubes and deliver great power to loudspeakers.
  • Tube element packaging improved, to the point of enclosing two or three different tube functions all in one vacuum envelope, thus saving space and expense.
1950sThis was the decade of the transistor, which was rapidly proving superior in all ways to the vacuum tube As one of the Bell Laboratories engineers said about the use of vacuum tubes: "It's like sending a freight train to bring home a pound of butter."
1958The last significant receiving tube development was the RCA Nuvistor, smaller than the end joint of one's little finger. It was a technological achievement, but once again, the tube was soon to be surpassed by the transistor.


Dhh:\Vacuum Tubes and Transistors\Vacuum Tube History.doc     3 Mar 2004


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