What's inside a 12AT7 vacuum tube?

A 12AT7 contains two triodes. 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 to the plate. They were designed almost 80 years ago for use in television sets, but make excellent detectors in vintage regen radios such as the Knight Space Spanner and the Lafayette KT-135.
12AT7 vacuum tube
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.
12AT7 vacuum tube
12AT7 vacuum tube
Something crazy happened when I cut the tube open. Notice in the left photo there is a shiny silver substance on the top of the tube. The is a "getter mirror" or gettering (not to be confused with "getter ring," which is the round thing on top in the right-hand photo). The getter is a substance on the inside of the glass that 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 the planet Earth.

It sucked the air out of my lungs, the garage windows imploded, my ears popped, snot shot out of my nose and a vortex of swirling wind nearly lifted the building off its foundation. Then, just like that, the getter vanished and all was calm. The top of the glass bulb was now transparent.
12AT7 vacuum tube
The getter mirror is gone. It sacrificed itself trying to maintain the vacuum after the glass was compromised.

12AT7 vacuum tube
12AT7 vacuum tube
Here's a Westinghouse 12AT7 that tests pretty bad. When it was cut open, the getter did the same thing, but you can see that it has turned a foggy gray. Why is this one gray? There were several substances that were used as getters, and the Westinghouse getter is probably different than the General Electric getter. The nipple at the top is where the vacuum was applied before being sealed off by melting the opening closed.
12AT7 vacuum tube
12AT7 vacuum tube
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. 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 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.
9 pin vacuum tube pins
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.
9 pin tube base
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.
12AT7 vacuum tube
12AT7 vacuum tube
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.
12AT7 vacuum tube
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.

12AT7 vacuum tube
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. In this case, it's 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 oxide material 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.
12AT7 vacuum tube
Now both plates are off.
12AT7 vacuum tube
You can see the filament peeking out of the top of the cathode. When the tube is in use, this makes an orange dot that can be seen glowing in the top of the tube. Actually, you see two because there are two triodes. The grid wires do not touch the cathode. A short circuit between the grid and cathode renders the tube inoperative.

12AT7 vacuum tube
12AT7 vacuum tube
The filament, cathode and grid, with a dime to show the scale. The filament has a white insulating material baked onto it. The metal cathode tube is made of nickel.

Notice in the right-hand photo that 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, but the filament can only give off so many electrons. The invention of the indirectly heated cathode was a milestone in vacuum tube development.
12AT7 mica
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. They were made in the USA, Great Britain, Holland, Germany, France and Japan. Where did they get all that mica??
Russian Mullard 12AT7
Genalex 12AT7
A Russian made Mullard 12AT7 and a Russian Genalex brand 12AT7. There is nothing "Mullard" or "Genalex" about them, except the names. An actual Mullard or Genalex would have been made in Great Britain. These are both made in the same factory in Saratov, Russia.
Chinese Shuguang 12AT7
A modern Chinese "Shuguang" brand 12AT7 available at Ali Express

Back in the day when tube counterfeiting was a big business (it still goes on today) you'd take a Sylvania, Philco or RCA tube and rebrand it as a British Mullard or German Telefunkin, but never the other way around. Nobody ever rubbed off the Mullard label and stamped "Philco" on it. I find this rather amusing.
Mullard Tube Boxes
New "vintage" Mullard tube boxes. Available on ebay. Be careful when you buy a 12AT7. A $7 tube can be turned into a $70 tube with a damp rag and a rubber stamp.

The 12AT7 is still being made today in Russia and China and you can buy a brand new one. Why would you want a new one? Of what use today is a vacuum tube designed in the 1950s for a television set? It would take FIVE BILLION of them to make a CPU used in a typical smartphone or home computer.

They are used in guitar amplifiers. People who play guitars have driven up the price of vintage 12AT7s to the point where it became profitable to manufacture new ones. The demand is over one million per year. A Russian Mullard is $40 and the Chinese Gold Lion is $60. An actual Mullard 12AT7 made in Great Britain can go for $120. In 1970, Lafayette Radio sold Mullards for $1.44 (equivalent to $11.25 in 2024).

Tube sellers who cater to guitar players extol the virtues of various brands. One site states, "If you are looking for clarity and headroom go for grey plates, if you would like more warmth and a richer overdrive sound go with black plates!" The color of the plates makes no difference,  "headroom" and "overdrive sound" are both ambiguous and subjective, and swapping vacuum tubes isn't going to make you a better guitar player.
McIntosh 12AT7s
12AT7 vacuum tubes in a modern McIntosh tube amplifier, illuminated from below to make them look cool. They are labeled "McIntosh USA" but they were actually manufactured in Russia. As far as I can figure, McIntosh is not named "McIntosh USA," so why stamp "USA" on the vacuum tubes? Why pull some stunt like this on a high end amplifier?

When I need to buy a 12AT7 for my radio I want reparations from guitar players, and I want Steve Hackett to deliver the check so I can get a picture of us together so it looks like I'm his friend or something.
Steve Hackett of Genesis
Like this, but the picture won't be fuzzy and instead of a CD it will be a check made out to me.