Vacuum Tubes (Valves)

Vacuum Tubes (Valves)

The vacuum tube (known as a ‘valve’ in the UK) is an interesting component that made many important technologies possible and it is still used in many things. There are many kinds of vacuum tubes for different jobs. Most types of vacuum tubes have been replaced by semiconductor devices, however vacuum tubes will occupy an important role in the future in the field of nanotechnology.

All ‘tubes’ basically consist of an anode and cathode in a gas or vacuum environment. They range in appearance from tiny ceramic parts the size of a corn kernel, to 1+ meter tall solid steel klystron tubes used in broadcast television or radar installations. Most tubes you can find around your house or garage in old electronics will be glass or aluminum, cylindrical, and between 1-6″ tall.  

1.) Crossing Barriers (The Edison Effect):
Thermionic emission is the concept behind most tubes. A filament heats up in the vacuum and when it reaches the right temperature electrons are emitted and cross over onto a positively charged plate. More on thermionic emission (wiki) >

So how is this useful?
You can create a diode with this concept.  A diode allows current to flow only in one direction. This can be used in devices to convert AC to DC power, convert voltages, or to construct a logic gate (which can do the job of acting like a relay or computer).  The first vacuum tube diode was made in 1904.
Other kinds of tubes like triodes can be used as amplifiers and enabled the development of the oscillator. Oscillators are used to filter and amplify signals, this is very important in radios, clocks, computers, television transmission, microwave/radar transmission and countless other things. In the field of radio/tv triode tubes replaced alternators as they could produce high frequency signals without the cumbersome mechanical apparatus.  Tubes were used in TV cameras and made the first video images. There are countless types of specialized tubes, some of which are still used today.

Sections:
1.) Edison Effect (How it works)
2.) Types of Vacuum Tubes
3.) Timeline
4.) The Future of Vacuum Tubes

Reliability:

Tubes have heaters and filaments that would normally burn out like light bulbs.  The ENIAC computer had more than 17,000 tubes and one would burn out every two days.  Consumers expected to replace tubes in their radios, TVs and other appliances. This is why radios and TV tubes had an easy to understand ‘plug-in’ design. Unique pin configurations prevented you from plugging in the wrong tube.
For many years there were thousands of different types of vacuum tubes with different ratings and functions.

Left: Consumers used to be able to buy all kinds of tubes for their devices.  The Edison Tech Center has many kinds of tubes in boxes and visible in old electrical devices.

2.) Common types of vacuum tubes:

Diodes
Triodes
Tetrode Pentode
Magnetrons

Right: Albert W. Hull with a dynatron (an alternate form of triode). Hull later developed the magnetron. The magnetron produces microwave energy and is the basis of radar and the microwave oven.

Thyratron
Thyratrons improved power systems, they could handle high voltages and act as a switch or rectifier. They could be used as a radio detector (in radar for example) and to convert AC to DC power. After 50 years they were replaced by thyristors (solid state).

Triodes:

Triodes are tubes which can be used in amplifiers, computers, radio receivers and transmitters and other things. Triodes were later replaced by transistors. This tube has a hot cathode in the center surrounded by a metal grid with the anode surrounding that. The cathode emitts electrons, and in the vacuum electrons freely flow through the grid to the anode. By energizing the grid negatively you repel more electrons, this means that less electrons can pass through the grid to get to the anode.

More on Triodes and their use in amplifiers in our page here >

Klystrons:

Klystrons were used in television, cold-war radar systems and radio transmission for many years, see our video with Rudy Dehn and colleagues at General Electric to learn about how they work.

Below: Video on how a klystron tube works

3.) Brief Timeline: History

1880 – Thomas Edison discovered thermionic emission when experimenting with lightbulbs, he found electrons left the carbon filament and were attracted to a metal plate placed in a bulb with a vacuum.

1904 -Diode vacuum tube invented by John Ambrose Flemming, based on Edison’s discoveries.

1910s – Irving Langmuir develops the first ‘high vacuum’ tubes using the newly discovered ductile tungsten. He worked with G.S. Meikle on the first Thyratrons at the GE Research Lab

1921 – Tubes made the first audio amplifiers work, so that the first loudspeaker could be developed.
Video – tubes used in the first loudspeaker >

1927 – Tubes make television possible, they are used in cameras, radio transmission and in the CRT television for display of the image.

Tubes in a radio on display at the Edison Tech Center

Above: Albert Hull, pioneer of tubes and many tube applications

Above: E.F. Alexanderson used tubes to pioneer TV/Radio broadcast systems

1940 – Left: this world war II era tube was used in radar and represented a new age in tubes. Tubes were getting smaller and were being made more durable. This magnetron was able to see farther and with greater resolution than previous radar. This Lighthouse Tube was made by A.D. McArthur and James E. Beggs at the General Electric Research Laboratory. During the war there was little time to test new tubes so tubes were made and shipped to the front lines for active testing.

Left: Lighthouse Tube used for high frequency applications

1947 – The ENIAC computer was built. This first computer for general purpose use used thousands of tubes. The computer (below) weighed about 30,000 kilograms. It was made of crystal diodes, relays, resistors, capacitors and 17,468 vacuum tubes.

Above: In the 1950s General Electric management was excited when they started producing high quality vacuum tubes which were a fraction of the size of past tubes. By this time solid state was beginning to make many tubes obsolete and the new revolution in tubes never reached fruition.

1950s-60s – Most vacuum tubes were replaced by transistors in the west.

1970s-80s Tubes are still used in many specialized applications like broadcast television and radio. By this point most tubes had been replaced in the west.

Left: In the 1970s Russia continued to use vacuum tubes for many applications. Tube-powered radar gave an advantage to the legendary MiG-25. The tubes were very powerful and not vulnerable to electromagnetic pulse. Tubes were more tolerant to cold temperatures than western electronics. Western jets had the added complexity of special environmentally controlled areas for electronics.

In the 70s and 80s tubes saw continued use in amplifiers and oscillators as they were more reliable at high power levels.

Above: what was an extremely expensive vacuum tube used in a professional video camera of the 1970s. There used to be one tube for each color with a prism to separate the red, green and blue. These tubes were replaced by charge-coupled devices (CCDs).

1990s-Today – Vacuum tubes are still used today.  Musicians still use tube amplifiers and claim they produce a different and desirable sound compared to solid state amplifiers.  See the video below showing the tube’s use in  a guitar amplifier:

Below: Corbin Irvin at the Edison Tech Center showing tubes in a typical audio amplifier.

4.) Future: The Nano Vacuum Tube

Vacuum tubes may make a comeback and replace standard microchips.  Engineers have been able to build a structure in phosphorus doped silicon and use nanotubes to build a switch.  These devices can operate 10 times faster than silicon transistors. Nano Vacuum Tube Articles:

Extreme Tech article >
Science Magazine article >

Let’s see what the future brings!

Computer Related Topics:

The first RAM
Early Computers at GE
Amplifiers
Hard Disk Drives
Computers
Capacitors
Computer Memory


Related Topics:
Semiconductor Electronics
Microwave Ovens & Magnetrons
Television
Amplifiers
Radio
More Stuff
Article by M.Whelan and W.Kornrumpf
Sources:
General Electric
AllaboutCircuits.com
Wikipedia
Semiconductors. Edison Tech Center. 2013
Diodes. Sparkfun
Return of the Vacuum Tube. Science AAAS. 2012
MiG-25 Foxbat. GolbalAircraft.org
John D. Harnden Jr. – former General Electric Engineer

Photos:
Edison Tech Center
General Electric