Vacuum Tube in 100th Year
Same Old Challenges

But what is a cold-cathode multipactor?

(Nuts & Volts, November, 2006)

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. 807Dealers 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 (, The Tube Store (, 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.


The hot-cathode vacuum tube is a challenge to engineers and builders because of its heat (mostly generated by that hot-cathode filament) and because of the filiment’s demanding power requirements. Blower-cooling is often necessary to dissipate waste heat. Large currents from heavy iron-core transformers are required to fuel the cathodes of these fragile, glass-enclosed devices. Also high voltages from special power supplies are required to fuel the plates: from 150 to 4000 volts of well filtered DC at a few hundred MA. Power supplies, and the transformers with which to build them, are expensive and hard to find.

De Forest power audion

The 811A requires four amperes and 1200 to 1500 volts on the plate. The handy little 6L6 power pentode takes 900 MA on the cathode heater and 250 to 360 volts on the plate. The 807 also takes 900 MA on the heater and requires 750 volts on the plate. The heaters of the one-kilowatt 4-400A suck a whopping 14 amperes with 4,000 volts on the plate. Its transformers are huge, ponderous, and expensive.

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 mag amp page.

Internal engineering

Appreciate the engineering challenges within the vacuum tube itself. Cathode temperature and plate potential are the key values in tube designing. The output (“emission”) of a hot cathode depends upon filament temperature. So it’s desirable to achieve the highest possible temperature. Yet one must engineer the tube so it attains a long life before the delicate filament inevitably burns out, and the tube becomes useless. So this sets limits to temperature.

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,

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. However, 19th-century electrical explorers, such as William Crookes and Nikola Tesla, energized high-vacuum tubes and globes with high-voltage oscillating currents, generated by induction coils and Tesla coils, with amazing results, giving not a thought to thermionic emitters.

The same hot-cathode mythology prevails in traditional incandescent and even fluorescent lighting. Tesla’s high-frequency lighting was cold-cathode as was his rotaing-brush detector. Tesla’s extensive research into vacuum electric-ray phenomena included the x-ray, which, unlike Roentgen, he produced cold-cathode. Tesla reportedly used a set of conventional vacuum tubes in the free-energy device that powered his ledgendary electric Pierce Arrow sedan, but these tubes were run cold-cathode. Apparently, the conventional tube can be run cold-cathode when high frequencies are used. So why is vacuum-tube radio based entirely on the hot-cathode?

Farnsworth’s cold-cathode multipactor tubes are high-frequency inventions made for radio oscillating and amplifying in practical transmitters. Their frequency 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.


Farnsworth says repeatedly in his patents that these devices are “over unity.” Regenerative feedback, and resonant reinforcement appear to be involved. That this device works without a hot cathode implies that there is plenty of electric energy that can be set free in a vacuum tube if it is properly stimulated. Free-energy seekers take note.

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.

Copyright © 2006 by George Trinkaus
All rights reserved.