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12AT7 vs 12AX7 Phase Inverter: Which Is Better? [2021 Guide]

If you’re curious about a 12AT7 vs 12AX7 phase inverter, this is the post for you!

I’m no expert on phase inverters, but I have been playing the guitar since 2003 and know a bit about this instrument and the gear that comes with it.

So let’s dive into phase inverters!

Phase Inverters

Tube amps are not just fragile but a bit complicated as well. But among all of their features and parts, we tend to forget about phase inverters. So what are they?

They too are somewhat complicated. Even some experienced techs have trouble fully understanding them. Generally speaking, a phase inverter is a device that changes a signal’s phase by 180 degrees. And in tube-driven amps, you have tube phase inverters.

A phase inverter also splits the signal from your guitar in two. One signal remains the same but the other is inverted.

But what does it mean to change the signal’s phase by 180 degrees? Well, you can think of your regular guitar signal as a continuous sine curve. It has its frequency and amplitude just like waves.

When you invert the phase, you essentially flip the waves upside down. The top is where the bottom was, and vice-versa. Here’s the graphic representation of a normal sine curve. This is a representation of your original guitar signal.

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And this next picture shows the original signal in blue and the inverted signal in red.

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Just bear in mind that, with guitar amps, there is no perfect phase inverter. These curves won’t be perfect in practice.

And your amp’s phase inverter is at the end of the preamp section. It feeds the signal to the power amp tubes.

Does a Phase Inverter Tube Affect Your Amp’s Tone?

So does a phase inverter actually affect the tone? The simple answer is yes. But if you were to hear your original guitar tone that’s phase inverted, you often wouldn’t notice any difference.

So what’s the deal? Well, if you push them both at the same time, it will be completely different. With a properly functioning phase inverter, you get a fuller and thicker tone.

Since it’s the last thing in the preamp circuitry, it helps push the power amp. This way, you get the power amp tubes to distort a little, which adds to the character of the tone.

If a phase inverter isn’t working properly, you’ll notice that your tone is thinner. So it has its practical importance in tube-driven guitar amps.

12AT7 vs 12AX7 Phase Inverter: Which Is Better?

Phase inverter use regular preamp tubes. However, in a lot of cases, you can find something else, like 12AT7. The standard 12AX7 and 12AT7 are interchangeable. So which one is better?

12AT7

Also known as ECC81, 12AT7 is a pretty common phase inverter tube. But it has a slightly lower voltage than the standard 12AX7. It’s mostly used for so-called push-pull amplifier circuits.

Not to get too geeky, but push-pull circuitry is common for higher output tube amps. In this setting, 12AT7 does a great job of keeping things sounding good. You won’t hear any unwanted odd-harmonic distortion.

At the same time, it has a stronger plate current and transconductance. In a practical sense, these traits make it suitable for producing high-ends.

The 12AT7 tube is known for giving less gain, less volume, and more headroom. As for more headroom, this means that you’ll be able to keep the tone cleaner. In other words, it is slightly more difficult for it to distort.

12AX7

12AX7 is your standard preamp tube. It has been present on the market since the 1940s. You can also find it in hi-fi equipment. It’s also very common as a phase inverter in guitar amplifiers.

The tube has a relatively high input voltage. However, it has a weaker plate current.

As a result of all of its features, 12AX7 gives a noticeably high output.

Which One Should I Use?

There’s no better or worse choice here. And it’s my understanding that the difference is minimal in most cases. The only thing that you will easily notice is the amount of noise on high-gain settings when you’re not playing.

But if you do care about these nuances, there some things to keep in mind. And, of course, the exact differences will differ depending on the exact amp model.

A phase inverter is the last component that the signal passes through before getting into a power amp. And the phase inverter tube properties change how the power amp reacts.

The main difference that we can notice is more gain with 12AX7. Or, in other terms, you have less headroom. On the other hand, 12AT7 has more headroom. And you can notice how it produces less gain.

With that said, it’s easy to recommend 12AX7 to anyone who prefers high-gain tones. And the 12AT7 tube will keep things slightly quieter and less distorted.

Of course, these differences are nuanced. If you’d like to keep things less distorted, I’d suggest using 12AU7. 

One thing that you might notice on some amps is that different phase inverter tubes can cause more noise. This happens in some cases when you push the amp’s gain knob near the maximum.

In this brief video below, you can hear how the amp reacts to different phase inverter tubes. Pay attention to the part with the maximum gain setting.

12AT7 vs 12AX7 Conclusion: Be Careful!

But at the end of the day, I think it’s best to go with the stock tube type. If your amp came with a 12AT7 tube, there’s probably a good reason for it. And if you’re planning to do some changes, please consult a professional beforehand.

Of course, you’re can change whatever you want with your amp. Just know that departing from the manufacturer’s recommendations can damage your amp.

If you want to read more about amp setups, check out my post about speaker cabinet setups.  

And, as usual, please feel free to comment below if you have questions about this or another guitar topic!

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