What are 12AX7 tubes in guitar amps?

If you're familiar with electric guitar tube amps, you know that most of the amps you see have something called a 12AX7 tube, usually used at the preamp stage. For those that want to read more about the difference between a preamp and power amp, you can do so in the following article.

Read more: Difference between a preamp and power amp

But what exactly are 12AX7 tubes? And why are they almost always used for guitar amps?

The simple definition is in the box above. They're vacuum tubes that produce high gain levels with a high amplification factor (high-mu).

We'll get into the more technical explanation here for those wanting to take a deeper dive.

Comparing Gain Levels in 12AX7 Tubes

There are a wide variety of brands and permutations of 12AX7 tubes. Here are a few of the companies that produce them:

• Electro-Harmonix (EHX)
• Fender (Groove Tubes)
• Genalex
• Tung Sol

These are just a few of the companies making 12AX7 amp tubes.

Amplification and Gain Levels

As we pointed out previously, all 12AX7 tubes have a high amplification factor. More specifically, they all measure at 100 mu. Technically, this should mean that all 12AX7 tubes would produce the same gain levels when placed in the same circuit. However, there's some variation between these tubes in primarily three categories:

• Gain levels
• EQ profile
• Noise levels

For example the Electro-Harmonix Gold Pin 12AX7 is quieter (less noisy) than the Tung-Sol 12AX7. In another comparison, the Tung-Sol Gold Pin 12AX7 has a higher gain rating than the JJ Gold Pin ECC803. 

When 12AX7 tubes have some variation in their EQ, they usually produce more low end, which is why tube amps tend to sound deeper and smoother.

You can read more about these differences in the following article:

Read more: 12AX7 tube comparisons by Amplified Parts

Amp brands that use 12AX7 tubes

A lot of amp brands use 12AX7 tubes, usually at the preamp stage. This includes combo amps, amp heads, and even isolated preamp-only units. We won't list all the brands but instead stick to the most notable.

• Marshall
• Fender
• Mesa/Boogie
• Diezel
• PRS amps

Different amps will have a different number of 12AX7 tubes in their preamp. For example, the PRS HDRX 20 amp head has only three, while the Diezel VH4 has seven.

So if you want an amp with 12AX7 tubes, here are the three decisions you should focus on:

1. Form of amp (combo, head, or preamp only)
2. Brand
3. Number of 12AX7 tubes

What about the ECC83 tube?

While there might be minor differences - like the differences between versions of the 12AX7 - the ECC83 tube is just the European equivalent of the 12AX7 and functions exactly the same.

In some cases you'll see amps that are powered by a mixture of 12AXs and ECC83.

Here are a few additional permutations of the ECC83:

• ECC803S
• E83CC
• E803CC

What about tubes for the power amp?

Guitar power amps are most often run by 6L6 tubes, which are more difficult to understand because of their technical complexity. But in simpler terms, they make a guitar amp really loud. This is why master volume, which is always placed at the power amp level, is controlled by 6L6 tubes. 

Conclusion

The 12AX7 tube is a go-to for guitar preamps. They've been around a long time and sound better than a lot of today's solid state amp circuits. If you have questions about the 12AX7 or something else we mentioned, feel free to get in touch via the comments section below.

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References

• TFA article on 12AX7 tubes
• Amplified Parts 12AX7 comparison
• Effectrode 12AX7 article