Writings
by George Trinkaus

The engineering challenges of the hot-cathode vacuum tube are the same today as when it was invented back in 1906: cathode heat, demanding power requirements, capacitance ...The inventing of new, more compact and efficient vacuum-tube designs proliferated madly up until the 1950's, but the art has been frozen ever since. Before Fleming's diode and DeForest's triode audion, inventors like Crookes, Tesla, and Roentgen had experimented with the magical effects of high voltage in vacuum. They created emissions of intense light and of strange rays, including x-rays. 

The "Fleming valve" was an evacuated glass envelope in which a light-bulb-style metallic filament, fed by low voltage and sucking high current, incandesces at 2000° F. This creates an electrical activity (or, in quantum parlance, "electron flow") that conducts unidirectionally through vacuum to another nearby element, a metallic plate charged to a high positive potential. Thus was born a new rectifier, and hence also a radio detector. Later it was discovered that electrical activities within a tube can be leveraged at will by applying small fluctuations to a metallic grid interposed between filament and plate (deForest). 

Thus human ingenuity created an amplifier, and hence a modulator, an oscillator, a regulator, and an electronic switch. Also created was a means for huge magnifications by regenerative feedback (Armstrong). Magnetic fields can influence electrical phenomena within a tube, as in the magnetron and the cathode-ray. The vacuum tube became the foundation for almost all of electronics for the next 50 years. Until the mass production of the transistor in the late 1950's, electronics was the vacuum tube.

Post-transistor, the conventional hot-cathode vacuum tube, despite its headaches for any engineer, builder, or user, has hung in there as a common choice for high-power-transmitter finals. You can number this writer among those who revere the 811 and the 807 for this purpose. For the ham or broadcaster's kilowatt rig, the big 4-400A often wins out against the clustering together of power transistors. 

The vacuum tube has seen a revival lately among audiophiles, who find in the vacuum tube ineffable qualities of depth, color, and warmth not found in solid state. I built a couple of Heathkit vacuum-tube hi-fi amps back in the 1950's, so I am sympathetic. But today, the audio freaks are making it more difficult for us transmitter freaks to find in the surplus market the old 811-A's, 807's, and 6L6's at cheap prices. Vacuum-tube dealers still carry huge inventories. Dealers say that among the most popular are the power tubes mentioned above, as well as the 6V6, 812, 12AX7, 12AU7, 12AT7, 300B, 6DJ8, 6922, and the 7308. 

Some vacuum-tube sources: Fair Radio Sales (fairradio.com), The Tube Store (thetubestore.com), Antique Electronics Supply, and R5D3 Surplus, Portland (503) 513-0410). 

With a few exceptions, vacuum tubes, are now almost completely out of manufacture in the USA. Dealers often go to Chinese or Russian sources, like Sovtek. The Russian military has a special respect for tube technology and uses it in the electronics of aircraft, tanks, and ships. Why? Because tubes are 10,000 to 50,000 times more resistant than solid state to destruction by an EMP.

Because of the transistor's moderate power requirements, and because of its compactness, electronics engineers pounced upon the invention as soon as it went into broad manufacture in the late 1950's.

Similarly, in the early '50's, the magnetic amplifier was seriously entertained, and even manufactured, as an alternative to the fragile vacuum tube. The mag amp is superior to tube and transistor in current and voltage capacities, more simple (you can build one yourself), and far more rugged in all respects. (See "The Magnetic Amplifier, a Lost Technology of the 1950's" in Nuts & Volts, Feb. '06, or Magnetic Amplifiers, Another Lost Technology from High Voltage Press.

The higher the plate voltage, the greater the electrical flow, but, at a certain high heat, there is a temperature-limitation barrier beyond which no increase in plate voltage will increase that flow.

Congestion of flow can also arise from an excessive build-up of electrical charge in certain spaces within the tube. This congestion sets a space-charge limitation to any conventional tube's performance. The proximity and shape of tube elements and the size and design of the envelope itself can enhance or mitigate space-charge problems.

Capacitance effects, especially at high frequencies, pose another design challenge. Each pair of tube elements constitutes a small capacitor that can affect amplification and oscillation. The larger the elements, the lower the frequency limit. Efforts to make the vacuum tube more compact created the miniature and "acorn" tubes.

A tube can self-oscillate due to internal capacity effects. Internal capacity is exploited in a positive way in the power-amplifier circuit called tuned-plate tuned-grid.

a cold-cathode tube?
In the 1930's, when some exploratory vacuum-tube engineering was still happening, Philo T. Farnsworth, obtained a series of patents for a cold-cathode vacuum tube.

Patent 2,091,439 (filed 1936).
More Farnsworth multipactor patents: 2,071,516, 2,135,615, 2,141,827, 2,172,152, 2,263,032, 2,174,487, 2,179,996. Found in Vril Compendium 10, Vassalatos, borderlands.com

 Farnsworth's over-unity cold-cathode
multipactor tube (1936)

lFarnsworth is known far and wide as the real inventor of television. (The history of radio is rife with rip-off: Tesla by Marconi, Fessenden by deForest, Barreter by deForest, Armstrong by deForest, Farnsworth by Sarnoff.)

A cold-cathode tube? But where does that necessary "cloud of electrons" come from?

Conventional tube theory, and the whole history of the amplifying tube's development, would have you believe that the only way electrical activity can be created in a vacuum is by "thermionic emission" from a hot cathode. (The same mythology prevails in traditional fluorescent lighting.) But Crookes didn't use hot cathodes in his evacuated spheres and tubes, nor did Tesla in his vacuum lighting devices or in his "rotating brush" detector. (Nor must one use a hot cathode to light a florescent tube, as shown Son of Tesla Coil.) Effects are achieved by disturbances within the tube caused by the application of high-frequency alternating currents, usually of high potential, generated by induction coils or by Tesla coils.

Farnsworth's cold-cathode multipactor amplifying and oscillating tubes are high-frequency inventions made for radio transmitting. Their range was 200 khz to 60 Mhz.

Says Farnsworth in his patent, "All that is necessary to set the tube into oscillation, is to energize the anodes, as there will be, in the space between the cathodes, a sufficient number of free electrons which are accelerated toward one or both of the cathodes by the potential of the anode, to strike thereon and cause the initiation of secondary emissions."

The tube is tuned to resonance in a way analogous to a tuned-plate-tuned-grid transmitter. Some of Farnsworth's other patents show the tube set into a magnetic solenoid.

Perhaps the invention's over-unity potential explains why this novel tube never made it into manufacture, despite the fact that it liberated vacuum-tube engineering from the problems of the old hot-cathode. Did the invention just fall between the cracks like so many others? (It did get a feature story in Radio magazine, October, 1934). Was the multipactor passed over because it was perceived on high as another "disruptive technology?"

I've built a mag amp, but I've never been tempted to construct a vacuum tube from scratch. However, reading these old patents makes me want to go shopping for a vacuum pump.

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