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What is a Vacuum Tube?

By Dan Blacharski
Updated May 16, 2024
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Before being replaced by transistors and integrated circuits, vacuum tubes (thermionic valves) were predominately used in electronic devices such as televisions, radios, and computers. They are still in use today in a few specialized devices.

The invention of vacuum tubes dates back to the observation of the so-called Edison Effect, an observation made by Thomas Edison. Edison noted that current flows between an incandescent lamp's filament and a plate within the vacuum, when the plate is connected to the positive end of the filament.

Because the first computers ran on vacuum tubes instead of today's small computer chips, a single computer had to contain thousands of vacuum tubes, and could fill an entire room. Early stereo amplifiers also used vacuum tubes, and even today, some audiophiles prefer them because they produce less distortion. Vacuum tubes are still used in some electric guitar amplifiers. Also, tubes are still used in some military applications, because tube electronics are not affected by the radio waves made by atomic explosions.

Vacuum tubes are typically enclosed in a glass enclosure, although some tubes use ceramic or metal instead. In its most basic diode design, the tube, or envelope, is sealed tight to create a vacuum. Electrodes within the envelope are attached to leads, which protrude out of the envelope and plug into a socket. A basic vacuum tube contains filaments within the envelope, similar to those of a light bulb. The filaments are heated and then release electrons, creating a negatively-charged electron cloud. The electrons are drawn to an anode, or small metal plate, within the tube, which is positively charged, and a unidirectional flow is established between the filament and plate.

An additional electrode in the form of a small screen-like grid is sometimes contained in the tube, which is then called a triode, which is more efficient and able to amplify the voltage. As voltage is applied to the grid, the flow between the filament and plate can be varied. In addition to diodes and triodes, further innovations followed, including tetrodes, hexodes, heptodes and octodes, designed for a variety of specialty applications and to minimize distortion. Some vacuum tubes combine the function of two or more diodes or triodes in a single unit.

One of the major drawbacks of vacuum tubes is that the filament becomes unstable over time. In addition, if air leaks into the tube, oxygen will react with the hot filament and damage it. The properties of a tube will change with age, which is why early vacuum tube television sets had to be adjusted often to produce a good picture.

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Discussion Comments

By anon341849 — On Jul 15, 2013

Vacuum tubes really produce less distortion. First, listen to a good tube amplifier.

By anon297983 — On Oct 17, 2012

Vacuum tubes do not produce less distortion. Transistors and vacuum tubes are both nonlinear. Traditional tube amplifiers are primitive. They rely on open loop gain without stabilizing negative feedback, and so they have terrible THD figures. (Add to that the nonlinearities in the transformers that are used to couple the power tube stages to the speakers.) Furthermore, the coupling to the speaker is poor: the amps have a high output impedance, which lets the speaker flop around at bass frequencies.

Audiophiles love distortion and ringing speakers. That's what makes the sound "warm", "punchy" with a "deep soundstage" and "shimmering details" that weren't heard before". When you remove these artifacts, the sound is "sterile, flat and cold".

A similar psychological effect takes place if you run your stereo with "rock and roll" EQ (bass and treble boosted) for a while and then switch to flat. It then sounds wonky in the midrange, like over the telephone, because your brain got used to the boosted bass. If you're used to high levels of distortion which adds brilliance and punch to the sound, and then hear high fidelity sound, you will hate it.

Vacuum tubes are still useful because they can amplify radio-frequency signals to large power levels. Transistors that do high frequency have small bases and do not handle a lot of current.

In every microwave oven, there is a vacuum tube called a magnetron which produces the fluctuating microwave-band electric field.

Of course, cathode ray tubes are vacuum tubes, and they haven't entirely died, and their demise is still rather recent history. The CRT continued to dominate well into the silicon age.

Vacuum fluorescent displays are still around, probably because they are gorgeous and "high tech" looking.

By indemnifyme — On Apr 28, 2012

I actually know a few audio people who prefer vacuum tube speakers. Like the article, they claim those kind of speakers produce less distortion. However, I never believed my friends when they said that!

These friends are the kind of people that prefer "classic" stuff. You know, the kind of people that would prefer to buy something on vinyl instead of download a digital file? They're often pontificating about the coolness of old school stuff, so I never believe them when they tell me something older is better. I guess in the case of vacuum tube speakers, they're actually right!

By Azuza — On Apr 27, 2012

@ceilingcat - I remember seeing pictures of the first computers when I took computer class, and it was totally mind-boggling. It just goes to show you how far technology can progress!

Anyway, it's funny that I came across this article, because I was reading another article a few days ago about something called a tube train. The article was talking about an airtight tube train that could reach speeds of 200 miles per hour.

And I think the airtight tubes that would be used for this train are the same vacuum tubes this article is referring too. I guess they have other applications besides computers!

By ceilingcat — On Apr 26, 2012

It's so crazy to think about how large the first computers were. The size of room! Meanwhile, I'm typing this on a laptop computer with a 15 inch screen that's small enough for me to carry around all the time. And my laptop is a hundred times more powerful than one of those room sized computers.

Still, I suppose there was no getting around it. They didn't have all the modern technology when computers were invented that we have now. You definitely could not make a laptop computer using vacuum tubes, that's for sure.

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