What Does a 12AX7 Do in a Guitar Amp?

A 12AX7 is a small dual-triode preamp tube. In a guitar amp it amplifies the very weak signal from your pickups, adds gain and a lot of the amp's character, and also handles jobs such as driving the tone stack and the power tubes. Most tube amps use several of them, and each one has its own task.

What is a 12AX7?

The 12AX7 is one of the most common tubes in guitar amps. Inside the glass are two separate triodes, so one tube gives the designer two amplifying stages. It has a high amplification factor of around 100, which makes it well suited to the first stages of an amp, where the guitar signal is tiny and needs a lot of gain.

You will also see it under other names: ECC83 is the European designation, and 7025 is a version originally selected for low noise. In practice these are the same tube type and fit the same socket.

What jobs does a 12AX7 do in a guitar amp?

Because each 12AX7 contains two triodes, one tube often does two different things. The common jobs are:

• First gain stage (V1). This is where your guitar signal enters the amp. V1 has the biggest effect on noise and microphonics, because anything that happens here is amplified by every stage that follows.
• Further gain stages. Higher-gain amps put several stages in a row. Each stage adds gain, and when a stage is driven hard it starts to clip. That is where preamp distortion comes from.
• Cathode follower. This stage adds no voltage gain but drives the tone stack with a low impedance. It is a key part of the classic British crunch sound and adds a slightly compressed feel.
• Phase inverter or phase splitter. In a push-pull amp, this stage splits the signal into two opposite halves to drive the pair of power tubes. When the amp is turned up, the phase inverter also contributes to the overdrive character.

Some amps use a tube for other jobs too, such as driving a spring reverb tank or buffering an effects loop. For those tasks designers often choose a tube with less gain.

12AX7 vs 12AT7 vs 12AU7

These three tubes look the same and share the same pin layout, but they have very different amounts of gain:

• 12AX7: highest gain, around 100. The standard choice for preamp gain stages.
• 12AT7: medium gain, around 60, and able to drive heavier loads. Often found in reverb drivers and some phase inverters.
• 12AU7: low gain, around 20. Used where a clean, low-gain stage is wanted, such as some effects loops.

Because they fit the same socket, some players swap a lower-gain tube into V1 to tame an amp. That changes the way the circuit was designed to work, so check with the maker or a technician first.

How do 12AX7s affect your tone, and when should you replace them?

The circuit around the tube decides most of the sound, but individual tubes do vary. Different makes can sound slightly brighter, darker, smoother or noisier, and the differences are easiest to hear in V1.

Preamp tubes usually last much longer than power tubes, often years. Signs that one needs replacing include hiss or crackling that does not go away, a ringing or squealing sound when you tap the amp (microphonics), or a clear loss of gain. Our article on when to replace amp tubes goes through the signs in more detail.

Only handle tubes when the amp is off, unplugged and fully cooled down. If reaching a tube means removing the chassis or opening the amp, leave it to a technician: there are high voltages inside a tube amp, even when it is switched off.

How 12AX7s are used in Kool amps

The Marvell, the 1984 and the Ace each use 3 x 12AX7 preamp tubes. There is no 12AU7 in these amps.

The Ace shows nicely how three tubes cover the whole signal path. Its 12AX7s work as the gain stage (V1), the cathode follower (V2) and the phase splitter (V4). With a single channel and an ultra-wide gain range, it goes from sparkling cleans into crunch as you turn up the gain.

The effects loops on Kool amps do not use a tube at all. The loop is designed to be ultra transparent, so your pedals and rack effects do not colour the original tone. You can read more about how that works in series effects loop explained.

Not every amp is built the same way. The Blue Sky, Kool's pedal-platform amp with tube-driven spring reverb, uses a combination of 2 x 12AT7 and 2 x 12AX7, because its design is focused on clean headroom and reverb rather than gain. And if you want to know how the preamp tubes relate to the power section, see preamp vs power amp distortion.

Your next step

If you want an amp where the preamp tubes give you a wide range from clean to crunch on one channel, start with the Ace. If you want two channels with Plexi-style clean and high gain, look at the Marvell. Questions about tubes in your own Kool amp? Contact Kool with your model or serial number.