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Confused about which tubes to get?
What are the differences between 12AX7 vs 12AT7 vs 12AU7 tubes?
Learn common applications and options for swapping tubes.
Tubes, or valves as some call them, are found in all manner of electronic equipment, from guitar amplifiers to boutique preamps, and there are even tube-powered microphones. While there are hundreds of different types of tubes, a select few tend to be used in pro audio equipment.
However, even those few types that get used frequently in audio equipment can still be confusing, so I’m here to help clear things up for you, specifically with three of the most commonly used preamp tubes, the 12AXY, 12AT7, and 12AU7.
12AX7 vs 12AT7 vs 12AU7: Differences Explained
The biggest difference between these power tubes is the amount of gain they will provide. The 12AX7 has a gain factor of 100, while the 12AT7 has 60. The 12AU7 has the smallest, at just 20.
The lower gain of the 12AT7 and 12AU7 also allows them to have a higher headroom, which can provide a warmer and cleaner sound.
What Is Gain/Amplification Factor?
The amplification factor µ of a triode valve / vacuum tube is a measure of the relative effectiveness of the grid and anode voltages in producing the electrostatic fields at the surface of the cathode.NME
The 12AX7 tube is probably the most common tube used in any audio equipment. It was first introduced in the 1940s and has been in almost continuous use and production in all manner of audio equipment developed since then. It’s unusual to find any tube guitar amp without at least a single 12AX7 in it, most commonly in the preamp.
The 12AX7 is known for its very high gain capabilities (100 times the initial input voltage), and its small size and cheap production cost has also contributed to its longstanding use.
Quite simply the best 9-pin triode preamp tubes you can buy - specifically designed and tested for musical instrument amplification and standard equipment on Fender amps. Outstanding and consistent performance for mixing and matching to get a signature tone from your amp.
The 12AT7 tube is almost identical in looks to the venerable 12AX7 tube, and in many ways can be used for the same applications as a 12AX7. However, the 12AT7 features a somewhat lower gain (typically about a factor of 60-70) and runs at a higher current.
What does this mean for the sound? A 12AT7 tube will give you slightly less volume, lower gain, and a higher headroom than a 12AX7 tube. If you’re looking to make an amp sound warmer or just have a little bit of extra headroom, you can try swapping a 12AX7 for a 12AT7 and see if you like the results more.
The 12AU7 tube also looks similar to 12AX7 and 12AT7, but its applications are typically very different.
12AU7 tubes have very low gain (typically only about 20), which gives them a great amount of headroom and a much higher current.
This is typically not enough amplification to actually drive a guitar signal enough to sound good. Typically 12AU7 tubes are used in tube-driven effects within guitar amps (such as spring reverb or tremolo effects in Fender amps) and are commonly used in microphone preamps and tube microphones.
While you can physically use a 12AU7 tube in place of a 12AX7 in a circuit, it’s not recommended to do. Most circuits that are designed for a 12AX7 will not react well to the significantly lower gain of the 12AU7. It can also cause issues with the bias of the amplifier, which can lead to very serious damage, including blowing the amp circuit!
What Does A Tube Do?
Without getting too far into the physics of it, tubes are electronic devices that amplify the electrical current of your signal. Another way to think about: consider a small, low wattage tube guitar amp (like a Fender Champ) and a large, high wattage guitar amp (like a Mesa/Boogie Dual Rectifier).
A Fender Champ only has 1 tube in the preamp section, and a total of 3 tubes in the entire amp circuit, while a Dual Rec has 5 tubes in the preamp section, and 11 in its entire circuit. A Champ doesn’t put out much volume, and is only rated at 5 watts, while the Mesa is rated at 100w and will easily shake your entire apartment floor and irritate the neighbors when you crank it up.
(If you want to go loud without creating a fuss, check our guide to the best amp-in-a-box pedals for getting hot amp tones without the volume).
Essentially, more tubes = more power = more gain and output volume.
This generalization may not always apply, but for the purposes of this article, it’s good enough!
In the circuit of a guitar amp, there are two major sections where you will commonly find tubes. The first is in the preamp section, which is where the very low output signal from your guitar is amplified. From there, the signal can be sculpted by the amp’s equalizer section, and you can control the overall volume from the input with the gain control.
The other is in the power amp section, where the signal is amplified even further to actually project sound out of the amplifier through a speaker. Since these two different applications require vastly different amounts of amplification, different tubes are typically used.
12AX7 tubes and their variants are most commonly used as preamp tubes.
In other tube-based pro audio gear, such as standalone tube preamps, the tube operates in the same manner as in the guitar amp mentioned above, but instead of being connected to the guitar amp’s power section, the tube preamp is connected to a mixer or your audio interface.
What Is The “Best” 12AX7 Tube?
Many people like to ask the simple question of “what’s the best 12AX7 tube?” But the answer is not so straightforward.
As mentioned above, the various different brands and variants of tubes are generally designed for different objective applications more so than of differing “qualities.” It’s also important to note that pretty much all tubes used in audio equipment come from one of 3 different factories that all produce the tubes for every brand.
The actual difference between brands comes down to the testing that each manufacturer does to ensure that quality control between different tubes of the same brand/label are within their set tolerances.
Each company does hold their tubes to different levels of QC, which can result in the various “good” and “bad” tube brands that you see all over guitar forums.
Ultimately, you should do what makes the most sense for your financial situation! Don’t let naysayers lead you astray in the idea that only the most expensive tube is good enough, because in 99% of situations you’re not going to actually notice!
Are 12AX7 and 12AU7 Interchangeable?
Technically, yes, you can physically interchange a 12AX7 tube for a 12AU7 tube in your amp without risk of major damage to your amp. However, it’s usually not recommended to do so, because the huge difference in gain between the 2 tubes usually won’t give you a very good sound (and it’ll be significantly quieter than it was before).
The other issue you can run into is the fact that the 12AU7 has a higher plate voltage than a 12AX7, and this difference can cause a host of problems in your amp circuit that was designed around the use of a 12AX7.
So really the answer is technically yes, but you still should not swap them!
Do Preamp Tubes Affect Tone?
It is absolutely true that guitar preamp tubes affect the overall tone and response of your guitar amp (or any other audio equipment for that matter). Different brands tend to design their tubes for different applications, even for the same model numbers.
Some 12AX7s are designed for the absolute highest amount of gain possible, while others are designed to last a long time or be the most durable. It’s important to try out different brands of tubes in your own equipment to see which ones you like best.
In most guitar amps, the most significant tonal change from a tube will come from the V1 guitar preamp tube. This tube is the first thing that your guitar hits in the amp, and it has a significant role in shaping the overall sound of your signal.
One of the most common things to experiment with is swapping the V1 tube that comes with your amp with another. This is almost always going to be a 12AX7 tube, but it ultimately depends on your amplifier’s circuit, so it’s important to double-check your amp or its schematic to make sure!
Another common swap that guitarists make is to swap a 12AT7 tube for the commonly used 12AX7 used in the V1 position. If you’re looking to give your amp a little bit more clean headroom or want your tone to sound warmer, this is a super easy and simple “mod” you can do that’s about as easy as changing a lightbulb.
With that being said, I’ve never tried this out to see if it makes a significant difference, and the extent to which it does affect your tone is always going to be subjective.
While this article only scratched the surface of the various intricacies of guitar amp preamp tubes, we’ve hopefully brought some understanding to the major differences between the various common preamp tubes you’ll find in most guitar amps today.
With some careful research and planning, you can find the right tubes for your amp that won’t cause any problems in the long run. As always, please rely on a professional to install them, unless you are absolutely sure what you are doing!
(Are you new to guitar or just want to improve your skills? Check out our reviews of JamPlay and Guitar Tricks if you want to learn online with the pros!)