All about 12AX7 Tube
What is a 12AX7 tube?
12AX7 tube is a small signal dual-triode vacuum tube found in just about every tube gear today.
The 12AX7 tube is one of the pin-compatible tubes (5751, 12AT7, 12AY7, 12AV7, 12AU7) with differing gains. Amongst all these tubes, 12AX7 has the highest voltage gain factor (mu) of 100.
The high gain factor and the dual-triode configuration allow the tube amp designer to build an amplifier with fewer components.
The 12AX7 tube is used in practically all guitar and audio tube amps today for this reason.
12AX7 Tube by Many Names
TL;DR Current production 12AX7 can be named differently, but they all are directly interchangeable 12AX7 tubes.
In the golden days of vacuum tubes, 12AX7 was used in many different types of equipment. Since different types of equipment had different requirements, several versions of 12AX7 were created. The designations you often see are 7025, 12AX7A, 12AX7W, CV4004, ECC83, and ECC803.
In the old days, the designation codes varied depending on the application. For example, military and industrial applications had specific requirements, and variations of 12AX7 were designed to meet them. These tubes got special designation, and they were priced accordingly.
These days, however, these special designations don't mean anything, since there is no demand for specially designed 12AX7. Instead, the designations are used to differentiate the more expensive premium 12AX7.
12AX7 vs 12AX7A Tube
Initially, the 12AX7A tube (with an "A" suffix) had a controlled heater warm-up for use in a series filament circuit.
Series filament was a cost-saving technique used in cheap tube radios. It does not apply to modern tube gear.
In recent times, these suffixes don't mean anything anymore. Suffixes like "A," "B," and "C" are used for product marketing and differentiation.
For example, Sovtek has 12AX7WA, 12AX7WB, and 12AX7WC. These are near-identical tubes with slightly different characteristics. Also, Shuguang has used 12AX7, 12AX7A, and 12AX7B labels on the same 12AX7 tube.
Some tubes have the "S" suffix, as in ECC83S and 12AX7LPS tubes. In these cases, "S" indicates the tube has spiraled heater filament. Spiraled heaters are lower in hum and more desirable.
12AX7 tubes produced today all use spiraled heaters, however. So having an "S" suffix doesn't mean anything special anymore.
When it comes to 12AX7A, 12AX7B, and other suffixes on current production tubes, you can consider them part of the product name and not worry about it.
12AX7 vs 12AX7B Tube
As mentioned above, the 12AX7B designation is purely for product marketing. For example, Shuguang used to mark their tubes as 12AX7A, and then they started marking them as 12AX7B. The "B" suffix doesn't mean anything, except that it is the version after 12AX7A.
What is a 7025 Tube?
7025 is a low microphony noise version of 12AX7. 7025 is a 12AX7 that doesn't pick up vibration noise.
Vintage Fender amps used 7025 in high gain stages to avoid unwanted feedback and noise.
Current production 12AX7 tubes are always used in audio or guitar amps, and they are designed to be equal to or better than the original 7025 tubes.
Some current production tubes are marked as 7025, but this is mostly marketing. Any of the current production tubes will work in place of 7025.
One exception is Electro-Harmonix 7025EH. This tube was designed to be extremely low noise and live up to its 7025 type designation. If you have any concerns about microphonics, 7025EH is the tube to use.
ECC83 vs. 12AX7 Tube?
ECC83 was the European designation for 12AX7. There is no difference otherwise.
What is an ECC803 Tube?
ECC803 was a type code given to a special high-reliability 12AX7. The original ECC803 was usually a frame grid design. This made it very special.
The frame grid was one of the last great inventions in vacuum tubes. In a normal 12AX7, the grid wire is wound around two posts. On a frame grid tube, the grid wire is wound on a rigid frame. Frame grid tube has very low microphony and tight tolerance.
Modern tube manufacturers have resurrected the ECC803 type code as premium versions of 12AX7. They are not a frame grid ECC803, however.
12AX7 is a Dual Triode Tube
The 12AX7 actually has two vacuum tubes in one. Each 12AX7 contains two triodes. We call this dual-triode.
What is a triode? You can think of a triode (or any vacuum tube for that matter) like a faucet valve. In a faucet, the high-pressure water flow is turned off/on by a valve.
In a vacuum tube, the electron flow is turned on/off by the signal applied to the control grid.
The English refer to vacuum tubes as "Electron Valve" for this reason. Well, that and "Tube" already referred to subways in the UK.
A 12AX7 tube contains two of these triodes, each with a mu of 100.
What is Matched Triode?
Due to manufacturing tolerances, the two triodes in a 12AX7 tube are not always the same. The triodes can vary quite a bit in some 12AX7.
The unmatched triodes are not an issue in most circuits. In some cases, however, using a 12AX7 tube with matching triodes is beneficial.
An example is a phase inverter, which drives the power tubes. In a phase inverter, one of the triodes drives one power tube, and the other triode drives the other power tube.
You do not want a 12AX7 with two triodes that are wildly different from each other in this case. Of course, this is not just with 12AX7. The same applies to any other dual-triode tubes like 12AT7.
We offer matched triode screening options on all dual-triode tubes. A matched triode tube is ideal for use in the phase inverter circuit.
What is Microphonic Noise?
Tubes can pick up vibrations and output them as signals. This phenomenon is called microphony, as in microphones. When tubes pick up vibrations, electrodes vibrate, and the distance between them changes, modulating signals like microphones. Please refer to Microphonic Noise for more details.
Microphony is inherent to all vacuum tubes, and all tubes are microphonic to some extent. With a high-gain tube such as 12AX7, the effect is more pronounced.
If you have a very high gain amp, it is better to select tubes that are less prone to microphony to reduce the chance of running into problems.
In general, some tube designs are less susceptible to microphony than others. Examples are TungSol 12AX7 or Mullard CV4004. These tubes have short plates that are less likely to pick up vibration. And, of course, you'll never have to worry about microphonic noise if you go with Electro-Harmonix 7025EH.
Use these tubes if you are concerned about microphonic noise.
We also offer a Low Noise Screening option if you want the quietest tubes.
12AX7 Substitute
There are several tube types that can be used in place of 12AX7. You can use these substitute tubes to change the tone of your guitar amp. You can read all about it on the 12AX7 Substitution page.
How Long Do 12AX7 Tubes Last?
A 12AX7 tube usually lasts a very long time. 1 to 5 years or more is not unreasonable.
A tube wears out mainly due to heat. The cathode that emits electrons will get old and tired with use, and higher heat accelerates this wear. Fortunately, 12AX7 is usually used with a very low plate current, so a 12AX7 in a gain stage does not dissipate much heat and can last a long time. 5000 to 10000 hours is a good rule of thumb to replace.
There are rare cases where a 12AX7 will become microphonic as it ages. The likely explanation is that thermal cycling causes some internal components to loosen or shift and become more vibration prone. Once a tube goes microphonic, you can only replace it with a new tube. For this reason, it is a good idea to carry spares if you are a gigging musician.
As a side note, power tubes like 6L6 and EL34 tubes tend to last much shorter than 12AX7 due to the heat they generate. EL84 tubes wear out even quicker since it runs even hotter Power tubes should be replaced more frequently than a 12AX7. If an amp is used regularly, the rule of thumb is six months to a year. Please refer to How Long Do Power Tubes Last? for more information.
Why does a new 12AX7 test bad on some testers?
Current production 12AX7 will often show as bad on Emission testers, even if the tube is perfectly fine. The tubes are 100% good. This is because of the way Emission testers test these tubes. Refer to Emission testers and 12AX7 for more information.