12AX7, 12AT7, & 12AU7 Preamp Tubes differences explained

What is the Difference Between the 12AX7, 12AT7, and 12AU7 Preamp Tubes?
Let’s take a look at the less glamorous, but equally important type of amplifier tube – the preamp tube.
The 12AX7, 12AT7, and 12AU7 all belong to the same family of nine-contact, twin triode tubes. In many cases, these tubes are interchangeable and can be easily swapped. We will explore why you might, or might not, want to swap out the different types of tubes.

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The 12AX7

The 12AX7 is by far the best-known preamp tube among guitar players. It has a gain output of 100, making it perfect for the preamp stage of an amplifier, allowing it to drive the power section harder and enabling it to reach high levels of distortion. The 12AX7 is also used in many audio preamps and guitar effects pedals. There are many versions of the 12AX7 tube from various brands. The 5751 version is a military version that is rugged and has a lower gain of 70.

The 12AT7

The 12AT7 is another well-known tube among guitar players. It has a much lower gain output of 60, but what you lose in gain you gain in more headroom, higher fidelity, and increased current output. Because of their high fidelity, they are used in many tube microphone preamps, compressors, and recording equipment. They are also used in many organs. Due to their higher current, you sometimes see them driving effects such as Tube Driven Spring Reverb in many guitar amps. The 12AT7 has many different versions created for specific purposes, such as low noise and ruggedness. The 5965 version has a lower gain of 47.

The 12AU7

The 12AU7 has the lowest gain, with an amplification factor of 20. It is used much less as a preamp tube because of the low gain and more as a driver for the Tube Driven Spring Reverb. The 12AU7 is also commonly used in the Phase Inverter position of the amplifier because of the high fidelity and great headroom that it provides. The 12AU7 is used in many high-fidelity audio systems and recording equipment. The current output is sufficient to drive a pair of headphones or a guitar speaker. Consequently, it is used in many headphone amps, several small one-tube guitar amplifiers, and numerous home hobby projects. The 12AU7 is heavily used in all stages of organs. Many versions of this tube have been created, including low noise, military grade, and ruggedness variations.

As you examine each tube, you might notice that as the amount of gain decreases, the current increases. As the gain decreases, headroom increases, and fidelity improves. Noise is also reduced with lower gain tubes.

You can typically swap out 12AX7 tubes with 12AT7 tubes if you want a cleaner guitar tone with more headroom and don’t mind a drop in volume. This might be helpful for a jazz player or when using keyboards, vocals, or any other instruments with high frequencies that you don’t want distorted. The 12AU7 makes a less ideal swap for the 12AX7 because other factors such as bias come into play, and the mismatch can cause the tube to sound unpredictable and very quiet. You may need to examine your guitar wiring to get the proper resistance load necessary to use a 12AU7 in place of a 12AX7.

There are dozens of varieties of each of these tubes that span more than 60 years, and the sound that each creates will change over its life span. One of the most enjoyable aspects of tube amps is the never-ending quest for the perfect tone — that perfect tube. I encourage you to try as many tubes as you can and listen to the subtle differences, even in the same type of tubes, as this will inform you in ways that words and sound samples cannot.

If you have multiple tubes in the preamp section of your amplifier, the first one will always have the largest impact on your sound, so start there. This will usually be a 12AX7, but you can swap it for a 12AT7. Next, check your PI tube (in Push Pull type amplifiers) and try a 12AU7 or 12AT7 and see if you prefer the results to a 12AX7 tube. If you have tube-driven effects on your amp, try a 12AU7 or 12AT7 in place of a 12AX7 and see if you prefer the results. Experiment with any other tubes between the first one and the PI until you find your sound. Happy Hunting!

If you have used tube audio equipment or guitar amps in the past 50 years, you have likely seen at least one of these tubes in your device. They are mostly used in the preamp section or as a driver/control tube for vibrato or reverb. For instance, the Kalamazoo Model Two guitar amp uses a 12AX7 to control vibrato and another to drive the preamp.

You can affect the tone of your amp by swapping different brands of these tubes as well as different numbers in the 12A?7 family. The major difference between the 12AX7 and the others is the amount of gain or amplification that they create in the circuit.

Of all the brands, the Telefunken (Western Germany) is the most valuable and best sounding. It was used in high-end preamps and tuners back in the 50s and 60s. Other high-quality tube manufacturers of this style include Mullard (Holland), Amperex Bugle Boys (England), and Sylvania and TungSol (USA). Lesser quality manufacturers were RCA, GE, Sevetlana, Ruby, JJ, and Tesla.

The following chart shows the different amounts of gain for each of the tubes:

12AX7: Gain of 100
12AT7: Gain of 60
12AY7: Gain of 45
12AU7: Gain of 19

So what does this mean? – If your device calls for a 12AY7 and you replace it with a 12AT7, you will get more distortion or your device will be louder. The reverse is true if you replace a 12AX7 (gain of 100) with a 12AY7 (gain of 45); your device will be about 55% quieter.

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Difference Between 12AU7 and 12AX7?

What’s the difference between these tubes? From my reading, it seems the AX7 has higher gain. What’s the purpose of using an AU7 and AX7 for each channel? Are these tubes basically interchangeable?

Could I use 4 AX7s or 4 AU7s?

As far as pin outs are concerned, the 12 series tubes are all interchangeable. The 12AX7 has the highest gain at 100, the 12AT7 is slightly lower, and the 12AU7 has the lowest gain of the group. These are rough estimates as each tube will have its own electrical characteristics. Just swapping one (new) 12AX7 for another (new) 12AX7 could result in slightly higher or lower gain from the circuit.

Obviously, changing the tube type to a lower or higher gain will affect any circuit after that tube.

While it is generally fine to interchange the 12 series tubes, you should do so with some degree of caution. Inserting a higher gain tube in place of a lower gain tube could easily overdrive the circuits fed by that tube, resulting in distortion. Using a lower gain tube in place of a 12AX7 could deprive the following circuits of sufficient amperage, flattening the sound.

Lowering the gain with a 12AT7 or 12AU7 is the most common procedure, and its success depends on the circuitry it feeds. There are also other ways to reduce gain in most amplifiers.

There are also European numbers equivalent to all American tubes and military-use numbers for tubes that would otherwise qualify as generic 12AX7, 12AU7 or any other audio tube on the market.

There’s no definitive reason why designers might use a 12AX7 and a 12AU7 other than the tubes probably serve different functions in the amplifier. One might serve as the phase splitter and the other as the preamp or driver tube. These are "dual triodes" and there are two complete tube sections in each capsule. Therefore, one tube can serve both left and right channels depending on the layout of the amp. This is a more common practice with more modern tubes since tube costs have risen dramatically over the last few decades. Vintage amps from the Golden Age of audio likely have each channel fed by 1/2 of a separate tube since these amps were designed when tubes were inexpensive.

My advice is to stick with convention, as designers typically had various tubes available to them. There’s often a good reason why specific tubes were chosen for specific applications. To deviate from that is essentially attempting to redesign the amplifier.

A tube, particularly output tubes, changes its sound over time.

Consider why a manufacturer might choose a specific tube. Beyond the decision to use a 12AX7 here and 6DJ8 there, the choice is often influenced by availability, reliability, and sonic values. Tubes need to be available for the duration of production and for future repairs. Reliability is crucial to prevent early failures. High-end audio reviews now focus on stock products rather than tubes swapped during reviews.

For all tube products, warm-up time is important for optimal sound quality. Amplifiers should not run without a load, and the stability of the amp must be assured if left unattended for long periods. Standby circuits and soft start cycles help extend tube life by managing initial voltage stress. Small signal tubes have longer lifespans compared to power output tubes. Biasing arrangements also affect tube longevity. Extending tube life involves understanding its finite lifespan and the nature of wear over time.

In conclusion, knowing what you seek in terms of priorities and selecting the component or tube that satisfies those priorities is key. Spend time researching and be cautious in tube selection. Happy hunting and enjoy the pursuit of your perfect sound!

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