Modern electronics turn on as soon as you hit the power button. A true peice of vintage electronics, however, will take a few seconds to warm up because it uses a predecessor to the integrated circuit: the Vacuum tube, also known as the Valve, or Thermionic Valve. Now only found in high-end amplifiers and the occasional lab, these devices were the basis of most complex electronic devices in the days before transistors.
The mechanism of the vacuum tube is simple. There is a vacuum inside the tube, and a heated element similar to a lightbulb (called the filament, or the anode). Electrons fly through the vacuum to another electrode, called the plate (or, in electrical terms, the cathode) so the device conducts a current. This current only flows in one direction, so this forms what electricians call a Diode, one of the basic elements of an electrical circuit. This behavior means that an arrangement of vacuum tubes can be used to convert alternating current into direct current, which was their first use. But if you put a grid between the filament and the plate, the amount of current that the tube conducts depends on the voltage that is applied to this grid. In this form (called a triode), a vacuum tube acts as an amplifier, converting a small voltage into a larger one.
The vacuum tube was first developed in the 19th century by several inventors, including Geissler and Crookes, who invented the first Cathode Ray Tube (CRT), later used in televisions. The basic idea was further developed by many more inventors, including Edison (who patented the basic effect that makes them work, but failed to understand the process behind it), Tesla and many others. However, most of them failed to see any practical application for the technology, and it remained a lab curiosity until 1905, when John Ambrose Fleming developed and patented the rectifying diode, which rectified (or converted) high frequency radio signals, making wireless radio transmission and reception both possible and practical. If you are looking for the point in time when Steampunk turned into Cyberpunk, this is it.
Over the next few years, many more types of vacuum tubes were developed, creating more and more complicated functions in a single tube and created better and more accurate amplification. These included Pentodes, which added two more grids that created a more accurate amplifier response, and other more complex devices.
Eventually, vacuum tubes formed the core of the first computers, built around analog circuits and later, logic gates built from these glowing tubes. The first computer was Colossus, built by the British Bletchley Park codebreaking unit to help crack German and Japanese codes. The most complex version of this device used over 2400 vacuum tubes of various types, which had to be regularly replaced. After the war, the EDVAC (Electronic Discrete Variable Automatic Computer ) was the first true digital computer that ran a stored program, built by Bell Labs for the US Army with over 6000 vacuum tubes and 12,000 diodes in .
Vacuum tubes continued to be used by computers through the 1950s, with the last major model being the BRLESC (Ballistic Research Laboratories Electronic Scientific Computer), which was built by the US Army in 1960. Although it was much more sophisticated than the EDVAC, it used fewer tubes, because the tube makers had figured out how to combine multiple features into each tube. BRLESC used 1727 vacuum tubes and 853 transistors, which had just become commercially available.
Another type of valve was the photomultiplier, which amplify light. These devices use high voltage and a cascading set of electrical plates (called dynodes) to multiply small numbers of photons into a significant electrical signal that could be captured and recorded. As the technology improved, they could be used to capture video in very low light. These formed the basis of all video cameras until the popularization of the CCD in the 1970s, but photomultiplier tubes are still used by scientists who want to detect single photos created by experiments, such as in particle colliders and chemical reactions.
My personal favorite was the magic eye, which was used on a reel-to-reel tape recorder that I was given as a child. This primitive version of the VU meter used a glowing green bar to indicate the recording level, which was created by electrons striking the anode. The degree of illumination is controlled by another voltage source, usually tied to a signal meter or other measuring device. Although the tape recorder itself was, I think, a transistorized model, I remember being fascinated by this otherworldy green light that danced as I recorded the sound of me babbling into it. It had a sci-fi appeal that no standard VU meter or digital indicator could match. Although the technology was primitive, even by the standards of the mid-1970s when I used it, it looked futuristic and strange. And it still does.
This particular type of valve is still in use, although the devices that drive it have changed somewhat over the years. Many other people value the strange, alluring light that it produces, so these tubes still show up in high-end hi-fi and other devices, driven by digital controllers. However, these tubes should be approached with caution, as they are driven by high voltages that could kill. So while they are great to look at, don’t stick a screwdriver in there to see how they work.
This vacuum tube is one of the few survivors of a revolution in electronics that started in the late 1950s. The transistor was invented in 1905, but wasn’t practical to manufacture at the time. By the late 1950s the technology had developed to make them quickly and cheaply, and electronics manufacturers were quick to realize that a small,transistor could easily replace a bulky, power hungry valve. The smaller size and lower power requirements of the transistor meant that devices could be more easily driven by batteries, such as the portable radios of the 1950s and 1960s. The first of these was the Regency 1, a portable radio released in 1954 at a cost of $49.95. The company behind this was driven out of business by a wave of cheap Japanese exports in the 1960s, (including those from a small Japanese company called Tsushin Kyogo, later renamed to the more pronounceable Sony). The revolution had begun, and manufacturers never looked back, with valves becoming little more than novelties within a few years, only found on old devices that were being phased out. The vacuum tube was dead, and the transistor was king.
Well, almost. As I noted above, scientists still use vacuum tubes as instruments, and they are also prized by hi-fi enthusiasts, who prefer the warm, analog sound produced by these analog devices over their digital replacements. High-end amplifiers from manufacturers like Mcintosh labs use valves that were first designed in 1956, and retailers like Tube Depot offer kits for those who want to build their own amplifiers. Even large manufacturers like Samsung have a valve model in their lineups for hard-to-please (and wealthy) audiophiles.
High-end guitar amplifiers also still use tubes, because musicians also favor the analog sound that they bring to their work. Marshall amplifiers in particular are beloved of rock guitarists, and the company still makes valve amps, like the 2012 DSL series. However, even this bastion of analog technology is feeling the lure of digital: the company offers a line of amplifiers called the ValveState II, which the company claims will “give you a close emulation of the superb response and unique tonal feel of a valve amplifier, without using power valves.” Purists might baulk at this sort of claim, but using transistors is cheaper: you can pick up a 15-watt Marshall amp for less than $100, but a similar valve model will cost you just under $600.
So the vacuum tube is a gadget we miss because it paved the way for the transistor, the integrated circuit and the electronics that we take for granted.
It is fitting, perhaps, that vacuum tubes may be making a small comeback: in 2012, a team of NASA scientists at the Ames research lab announced they had made nano vacuum tubes. These nano tubes use the same principles as their larger ancestors, but on a minute scale, with a gap of just 150 nanometers (or about 0.00000056 inches) between the two main parts. In the paper that describes the technique, the writers estimate that they could bring this down to an incredible 10 nanometers with improved manufacturing techniques.
These nano tubes use very little power, and can run incredibly fast: up to 0.46 Terahertz at the moment, but the writers estimate that the will be able to get “well into the THz regime” with time.
So, in a few years, you might once again be able to buy a computer that runs on vacuum tubes, just ones that are a little smaller than the ones that your grandparents used.