What is the 12AX7 vacuum tube?

The 12AX7 is a type of small electronic tube. It has two parts inside called triodes. Triodes help make electric signals stronger. The 12AX7 tube is also called the ECC83 in Europe.

The 12AX7 tube was very popular in the 1950s and 1960s. It was used in many different electronic devices, such as radios, televisions, and audio amplifiers. Even though transistors and computer chips have replaced tubes in most new electronics today, some people still like to use 12AX7 tubes. They think these tubes have a unique sound when used in music gear.

Invention and history

The 12AX7 vacuum tube was invented in the late 1940s. After World War 2 ended, companies were looking to make smaller and better tubes than the ones used before. Engineers at RCA in the United States created the 12AX7 as an improvement on earlier tubes like the 6SL7.

The new 12AX7 tube could make signals almost 100 times stronger. This high gain was useful in many circuits. The tube also created less noise and had two triodes in one small glass package. Audio amplifiers, radios, and other devices quickly started using the 12AX7 in the 1950s because it worked well and didn’t cost too much money.

How vacuum tubes work

You might wonder how a small glass tube with no moving parts can make electric signals stronger. The triodes inside the tube can control the flow of electrons using electric fields.

A triode has three main parts inside:

1. A cathode that is heated up to over 1000 degrees F. This makes electrons fly off into the tube.
2. A grid which is like a small metal fence. Putting different voltages on the grid lets you control how many electrons can move through it from the cathode.
3. A plate or anode that collects the electrons that make it through the grid.

The electric signal you want to make stronger goes to the grid. This signal changes the voltage on the grid which controls the electron flow, making a copy of the signal that is bigger in voltage swings. Small changes to the grid voltage make big changes in the output electron flow and voltage.

Using the 12AX7 tube in circuits

The 12AX7 has two separate triodes inside. This lets you use it for different jobs in a circuit. Most of the time, the triodes are used one after the other to make the signal very strong.

Audio amplifiers and preamps have used the 12AX7 a lot. Famous guitar amps like the Fender Bassman and Marshall JTM45 had several 12AX7 tubes inside. The tubes would boost very weak signals from guitar pickups and microphones up to levels that could drive speakers. Home stereo preamps also used the 12AX7 for functions like volume control and tone adjustments by amplifying signals between stages.

12AX7 tube versions

Many different companies have made the 12AX7 tube over the decades. While they all pretty much work the same, some versions are more popular than others. This is because even tiny changes in the materials and construction can change the tube’s sound a bit when pushed hard in an audio amplifier.

Some of the famous 12AX7 versions include:

• Mullard 12AX7 “long plate” from the UK
• Telefunken ECC83 smooth plate from West Germany
• Amperex “Bugle Boy” 12AX7 from Holland
• RCA “black plate” 5751 (a 12AX7 with slightly lower gain)

These old versions from the 1950s and 1960s can sell for a lot of money today. Musicians and audio fans like how they sound and think they were made with better quality than newer tubes. But companies still make new 12AX7 tubes that work great too.

Controversy and debates

There are big debates about whether old 12AX7 tubes really sound different or better than new ones. Some people think any working 12AX7 will be basically the same if the circuit and parts around it are the same. Other people are sure they can hear small differences between tube versions when listening to music or playing guitar.

Some say the circuits the tubes are used in matter much more than the tube itself for the final sound. But many audio fans still look for their favorite 12AX7 versions. Arguments about the best 12AX7 tubes can start big fights on hifi and guitar forums online.

Do they still make the 12AX7 tube?

Yes, companies in Russia, China, Slovakia, and other countries still make 12AX7 tubes. Even though most electronics have moved to transistors and chips, some people like the sound and look of vacuum tube equipment. New tubes let people keep old radios and amps working. They also let companies make new audio gear that has the style of older tube designs.

Tubes are less efficient and need more power than transistors. They also wear out after a few years and need to be replaced. But when used in audio gear, many people feel tubes have a warmer and more pleasant sound, especially when turned up very loud. Guitar players often say that tube amps react to their playing in a way that feels more dynamic and touching.

So combo amps, microphone preamps, and other modern music devices still get made with 12AX7 tubes inside. The 12AX7 is almost 75 years old but it still has a big following and use today. Not bad for some glass tubes with parts that glow orange inside.

FAQ on 12AX7 tubes

Q: Can I replace a 12AX7 tube with a 12AU7 or 12AT7?
A: The 12AX7, 12AT7, and 12AU7 all share the same pin arrangement, so they plug into the same tube sockets. But the 12AX7 has a gain factor of 100, the 12AT7 is 60, and the 12AU7 is 20. Some guitar amps offer different input tube options for this – a 12AX7 input will be hotter and more overdriven, while a 12AU7 input will be cleaner. But in most circuits, it’s best to stick with the 12AX7 the device was designed for.

Q: How long does a 12AX7 tube last?
A: If treated well in a device that is working properly, a 12AX7 tube can last for many years, around 10,000 hours of use. But mechanical shocks, overheating, and aging can cause tubes to wear out faster and need replacement. The 12AX7 is not too expensive though compared to some audio tubes.

Q: Why are some NOS 12AX7 tubes so costly?
A: NOS means “new old stock,” 12AX7 tubes made decades ago but never used. Some brands like Telefunken have a reputation for great sound and build quality. Since they are not made anymore, unused tubes can go for high prices, over $100 each! But you can get modern production 12AX7 tubes that work great for $15-$25 each in most cases.

Conclusion

The 12AX7 is a small vacuum tube with a long history and influence. It was a key part of radio, TV, and audio technology for many decades. It lives on in music equipment like guitar amps, where people enjoy its sound and character. Understanding what’s inside this tube and how it works helps show the clever engineering that went into the early electronics we sometimes take for granted now.