Let's take a look at the less glamorous, but certainly 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 easily be swapped. We will take a look at why you might, or might not want to swap out the different types of tubes.
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 in guitar effects pedals. There are many versions of the 12AX7 tube and many brands. The 5751 version of the 12AXY tube is a military version that is rugged and has a lower Gain of 70.
The 12AT7 is another very well known tube among guitar players. It has a much lower Gain output at 60, but what you lose in Gain you get back with more headroom, higher fidelity, and an increased current output. Because of their high fidelity they are also used in many tube microphone preamps, compressors, and recording equipment. They are also used in many organs. Because of their higher current you sometimes see them driving effects such as Tube Driven Spring Reverb in many guitar amps. Just like the 12AX7 tube, the 12AT7 has many different versions created for specific reasons, such as low noise and ruggedness. The 5965 version of the 12AT7 has a lower gain of 47.
You might also like this HumbuckerSoup article: Preamp Tube FAQ – Frequently Asked Questions about 12AX7, 12AT7, & 12AU7
The 12AU7 is the last tube on this list and it has the lowest Gain, with an amplification factor of 20. This 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 used quite often 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 hi fidelity audio systems and recording equipment, and the Current from the tube is enough to drive a pair of headphones or a guitar speaker. Because of this, it is used in many different headphone amps, several small one-tube guitar amplifiers, and in many home hobby projects. The 12AU7 is also heavily used in all stages of organs. There are also many different versions of this tube that have been created for specific reasons, such as low noise, military grade and ruggedness.
As you look at each tube, you might notice that as the amount of Gain decreases, the amount of Current increases. As the Gain decreases, the amount of headroom increases, and fidelity is improved. Noise is also reduced with lower gain tubes.
You can usually swap out 12AX7 tubes with 12AT7 tubes if you want a cleaner guitar tone with more headroom and you don’t mind a drop in volume. This might be helpful to a Jazz player, and could also be helpful for using Keyboards, Vocals, or any other instrument with a lot of 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 look into your guitar wiring to get the proper resistance load necessary to use a 12AU7 in place of a 12AX7.
You might also like this HumbuckerSoup article: Power Tube Differences: EL84, EL34, & 6L6
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 the course of its life. One of the most fun aspects of tube amps is the never-ending quest for the perfect tone — that perfect tube. I encourage you to try out as many tubes as you can and to listen to the subtle differences, even in the same type of tubes, as this will inform you in ways that words and even sound samples cannot.
If there are many tubes in the preamp section of your amplifier the first one will always be the one that has 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 a 12AT7 and see if you prefer the results to a 12AX7 tube. Next, if you have tube-driven effects on your amp, try a 12AU7 or a 12AT7 and see if you prefer the results to a 12AX7 tube in this part of the amp. Now you can think about any other tubes between the first one and the PI. Just keep experimenting until you have YOUR sound, and…