12AT7

The 12AT7 vacuum tube was designed in 1947 for use in television sets, and was widely used in FM radio receivers, communication equipment, and industrial and military devices. They made excellent detectors in regen radio kits such as the Knight Space Spanner and the Lafayette KT-135. Specialized versions such as the 12AT7W and 6201 are resistant to vibration, and a version labeled 6679, which is resistant to fluctuating voltages, was made for car radios. (The "12" in 12AT7 is the filament voltage, so it was easily powered by an automobile's electrical system.)   No US manufacturer makes a 12AT7 today, but millions are made in Russia and China. They are mainly used as pre-amplifiers in audio equipment. The Russian tubes use the old names of Mullard, Tung-Sol, and Genalex Gold Lion, but the Russian and Chinese tubes do not  meet the specifications of an actual 12AT7.

An IEC Mullard 12AT7 as it was found in a Lafayette KT-135. It had been in place since 1968.

Photo by Vic Rodriguez

A Mullard 12AT7 as it was found in a Knight-Kit Space Spanner, where it had been since 1957.

The glowing cathodes in an Admiral brand 12AT7. It was actually made by General Electric, so the instructions on the glass to replace the tube with a genuine Admiral tube are comical. Some rebranded tubes, such as Emerson and Sears Silvertone, said to insist on the same brand.

A closer look at the tops of the hot glowing cathodes, which are hollow and extend downward. The white light on the tops is the glow from the filaments inside the cathodes. The outer glass can reach a temperature of 400+F.

In the United States, tubes labeled Admiral, Emerson, Sears, Motorola, Zenith, Delco, Lafayette or Realistic (Radio Shack) were actually made by GE, RCA, Tung-Sol, Ken-Rad, Sylvania, Westinghouse or Raytheon. Realistic "Lifetime" tubes were made in Japan and  Taiwan, and some 12AT7s were made in Great Britain. Tube manufactures even farmed their own tubes out to other companies. Philco tubes were made by Philco, Dumont tubes were made by Dumont, but some Philco and Dumont tubes were made by Sylvania. Above is a Japanese 12AT7 "registered" by Raytheon. What does that mean, if anything, and who made it? Ken-Rad made their own tubes, but in 1944 the employees went on strike. I don't know how it worked out for the workers, but the War Department seized the company. In 1945 it was sold to General Electric. Though the 12AT7 was developed two years later, you can find 12AT7s labeled "Ken-Rad". If you find a GE tube with the number 188 on the glass, it was probably made in the GE owned Ken-Rad factory.

A GE 12AT7 with the number 188 on the glass, made in the former Ken-Rad factory. Like many things in the vacuum tube industry, the "188 means Ken-Rad" rule is not always true.

Dissecting a 12AT7 vacuum tube

A triode consists of a cathode, which gives off electrons, a plate which attracts electrons, and a grid which controls the flow of the electrons from the cathode to the plate. A 12AT7 contains two triodes. Inside the glass is a vacuum, ergo the term "vacuum tube." Great effort is made to insure this is a "hard" vacuum and not even an atom or molecule of air is present inside.

Here's a General Electric 12AT7 vacuum tube. It's two tubes in one! One side isn't working very well, so this isn't good anymore. Near the bottom you can see a black line made with a magic marker. Let's cut it open along this line and see what's inside.

Something crazy happened when I cut the tube open. Notice in the left photo there is a shiny silver substance in the top of the tube on the glass. This is a "getter mirror," sometimes called a "getter flash" or "gettering." The getter absorbs (or gets) any stray atoms or molecules of gas inside the tube. When I cut the tube open the getter tried to absorb the entire atmosphere of planet Earth! With a loud hissing, papers and debris started flying around in the small tornado-like vortex of swirling wind desperately trying to enter the tube. Then, just like that, the getter vanished and all was calm. The top of the glass bulb was now transparent.

The getter mirror is gone. It sacrificed itself trying to maintain the vacuum after the glass was compromised. All rise. Let us bow our heads in silence. "Verily I say, he stood tall against molecules, but now his final battle lost, he rests eternally. Hail, Getter, your watch has ended." Please be seated.

Here's a Westinghouse 12AT7 that tested poorly. When it was cut open the getter got all it could get, and you can see that it has turned a foggy gray. Why is this one gray? Barium is primarily used as the getter, but it is sometimes mixed with magnesium and/or zirconium. The Westinghouse getter probably differs from the General Electric getter.

On the left, the GE has gray plates and a round, or halo, getter. The Westinghouse on the right has black plates and a square getter. The plates are made of nickel and coated with a graphite based substance which prevents electrons from bouncing off the metal and which helps dissipate heat. You'll sometimes see descriptions of the getters and plates when buying tubes. The color of the plates or shape of the metal that held the getter material makes no difference in the characteristics of the tube.

The pins are made in three sections. The part that goes through the glass has the same coefficient of expansion as glass. This keeps the glass from cracking during manufacture or when the tube heats up while in use.

After the pins are molded into the base, they are bent all at once in a jig designed just for this purpose. The connections from the pins to the tube elements (removed in this photo) are spot welded prior to the glass bulb being added and melted to the base.

Both the "getters" have a grooves in them that were filled with the getter material. Because the getter was "flashed" when the tube was made, what is left is the burnt residue. The getter itself went onto the inside of the glass. The getter is placed in the top of the tube so that it doesn't coat any of the metal parts inside the tube.

The getter ring is spot welded to one of the plates.

Now we can cut the supporting wire to the plate. It's called a plate because it actually was a plate in the first vacuum tubes. Electrons are given off by the cathode and are attracted to the plate. Someone got the good idea that two plates would be better than one, and that a cylinder would be even better than that. Then they realized it doesn't need to be a round cylinder, it can be flattened.

Under the plate there are two supporting copper wires that have a spiral of thinner wire of molybdenum wrapped around them. This is the grid. Inside the grid is the cathode, a hollow metal cylinder with a filament inside. The filament heats the cathode, which gives off electrons. The white fluffy stuff is a coating of material (barium, strontium and calcium carbonate) that has a huge amount of excess electrons. I don't know why it's flaking off, but it may be why the tube wasn't testing very good.

Now both plates are off.

You can see the filament peeking out of the top of the cathode. When the tube is in use, the filament makes the cathode glow red hot.  The grid wires do not touch the cathode. A short circuit between the grid and cathode renders the tube inoperative.

Once a precision piece of apparatus, now a small pile of junk. Eternal rest grant unto him, O Lord, and let electrons perpetually shine upon his plates. Amen.

The plates are small enough to fit on a U.S. dime.

The filament, cathode and grid, with a dime to show the scale. The filament has a white insulating material baked onto it, not to be confused with the white coating on the cathode, which is there to supply electrons. The metal cathode tube is made of nickel. In the right-hand photo the oxide material has flaked completely off. This is an "indirectly heated" cathode. In antique and low voltage vacuum tubes the filament is the cathode. The filament can only give off so many electrons, even if it's coated with a substance to improve electron emission, such as thorium oxide. The invention of the indirectly heated cathode in 1913 was a milestone in vacuum tube development.

The disks that holds everything in place are made of mica. In the mid 1950s almost 500 million vacuum tubes were being made every year in the United States. They were also  made in Great Britain, Holland, Germany, France, Japan, the USSR and China. Tubes were made in India with mica and other parts supplied by the British. Where did they get all that mica??