12AT7

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What's inside a 12AT7 vacuum tube?

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

Photo by Vic Rodriguez

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. Many rebranded tubes had a similar message. Some tubes, such as Emerson and Sears Silvertone, said to insist on the same brand. 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 by Matsushita in Japan. Philco tubes were made by Philco, but also by Sylvania. Rebranding in this manner is not counterfeiting.

A closer look at the red hot cathodes inside the 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. It was designed in 1947 for use in television sets, but make excellent detectors in vintage regen radios such as the Knight Space Spanner and the Lafayette KT-135.

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 the "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 the planet Earth. It sucked the air out of my lungs, the garage windows rattled, my ears popped, 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.

The getter mirror is gone. It sacrificed itself trying to maintain the vacuum after the glass was compromised.

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.

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.

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. 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.

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.

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.

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, Japan, and the USSR. Where did they get all that mica??

A Russian made Mullard 12AT7 and a Russian Genalex brand 12AT7. There is nothing "Mullard" or "Genalex" about them, except the names. Mullard stopped making vacuum tubes in 1982. These are both made in the same factory in Saratov, Russia.     Gold plated pins! On the left is a used, vintage Amperex tube with gold plated pins. On the right is a Russian Genalex, also with gold plated pins. Just plugging the Russian tube into a socket scraped off the gold. It looks more like gold paint over rust. Whatever the gold colored substance is on the Russian tube, it's probably useless. (Pictures from ebay)     Radio Shack "Realistic" brand tubes also had gold plated pins. Realistic tubes cost more, but were "guaranteed for life." For example, in 1970 a 12AT7 from Lafayette was $1.44. A Radio Shack "Lifetime" tube was $2.19, but if it went bad you got a replacement for free. That sounds like a good deal, but the guarantee didn't mean Radio Shack would replace the tube forever. The "lifetime" was the "life of the apparatus in which it is used." Radio Shack began to offer "Lifetime" tubes when radios and televisions that used vacuum tubes were becoming obsolete, replaced with transistorized units. Many of these tubes weren't in service long enough to go bad, so it was a good deal - for Radio Shack. By the way, $2.19 in 1970 is $17.81 in 2025.

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.

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, a rubber stamp, and a Mullard box.

In 2024 I visited the Ali Express website and bought the cheapest 12AT7 I could find, a Chinese PSVANE brand labeled ECC81 (the European designation of a 12AT7). The diameter is slightly larger than a vintage 12AT7. The price was $13.74. The box is a little too "elegant" for my liking. I think it looks like a personal care product came in it. "Honey, I've got that rash again. Can you get me the Pissvane from the medicine cabinet?"

There is an unusual structure near one of the plates. It holds up a post that has the getter ring attached to it. I went back to Ali Express to see what I had ordered, and none of their ECC81 tubes had this thing in the pictures.

Now for the test! Both triodes of a new 12AT7 should read at least 4000 on the middle scale of this meter. One triode of the PSVANE tube read 2800 and the other one gave a reading of 3100. A 12AT7 should read 3800 after 500 hours of use at the maximum plate voltage of 250 volts. This new one from China seems like it's been in use for a thousand hours already. The Chinese must be selling the rejects of their "premium" versions. Fortune cookie say, "Gullible man buy cheap tube." A 12AT7 should test like a 12AT7 no matter how much it costs! In September of 2024 I bought a used Admiral for a dollar that tests better than this. (It's the one at the top of this page.) PSVANE also makes a version that they sell for over $100. Do you want to take a chance on it? Not me!

Why is the 12AT7 still being made today in Russia and China? Why would you want a new one? Of what use today is a vacuum tube designed in the 1940s for television sets? It would take FIVE BILLION of them to make a CPU used in a typical smart phone 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. A vintage Mullard 12AT7 made in Great Britain can go for $120, as does an Amperex made in Holland. 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 whatsoever, "headroom" and "overdrive sound" are both ambiguous and subjective, and swapping vacuum tubes isn't going to make you a better guitar player.

12AT7 vacuum tubes in a modern McIntosh tube amplifier, illuminated from below with green LEDs. 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? Are they are intentionally being deceptive, pulling some marketing stunt to sell an expensive 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.

Like this, but the picture won't be fuzzy and instead of a CD it will be a check made out to me. Photo taken at the Sellersville Theater, Sellersville, PA

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