If you own a tube based guitar or bass amplifier or even a Hi-Fi audio amplifier, chances are you will find a 12AX7 under the hood. Maybe even a few of them.
The 12AX7 is the first choice for tube amplifiers because of its high gain and relatively low noise characteristics.
The 12AX7 vacuum tube was developed by RCA to replace a tube called the 6SL7. The aim was to design a tube that was smaller, less microphonic, less noisy and more sturdy.
This was especially important in audio applications and was released for sale to the public on September 15, 1947.
And even though it is as popular as it is, most people know very little about them. Even changing a 12AX7 tube can be a nerve racking experience.
So we are going to change all that with this ultimate guide! We suggest you bookmark this article so you can return to it when you need to know about the 12AX7!
12AX7 In Rock History
When you think of some of the amplifiers that powered the music scene of yesterday, you probably think of brands like Marshall or Fender.
These are iconic amplifiers for sure, but what about the mighty tube that made it all happen? That’s right, the 12AX7 has been used by amplifier manufacturers since the early 1950s!
These little tubes were the driving force of rock and roll sound from pretty well the beginning.
Yet there is little to no mention of it even though it is one of the cornerstones of guitar sounds!
Even today amplifier manufacturers use the 12AX7 because it continues to be the sound in a players head. It’s time we give the 12AX7 some well deserved recognition!
What Is A 12AX7?
The 12AX7 is a 9 pin dual triode, meaning it has 2 sides or amplifiers in one glass envelope or tube.
The 12AX7 is a small signal high gain amplifier. It can take a small input signal and amplify it a great deal.
This is especially handy in instrument amplifiers as the signal from instruments is very low. Normally in the milli-volt range.
Each triode or amplifier inside the tube have 3 pins each that control the amplifier and a shared heater element.
This heater element stimulates electrons inside the tube and is seen by a faint orange like glow.
The 12AX7 is a part of a large family of twin triode vacuum tubes each with their own characteristics.
They span a wide range of voltage gain, ruggedness, microphonics, stability and even lifespan.
But the 12AX7 is the most commonly used tube of them all with over 2 million being produced annually.
12AX7 Technical Information
12AX7 Specification And Max Ratings
The label 12AX7 is a series of designators that describe what the tube is capable of and how it is to be used. It is not simply just a model number.
For example the 12 indicates that the heater will require 12 volts.The A indicates it is an amplifying device.
The X indicates its electrical characteristics while the 7 indicates the number of active pins.
Filament Voltage: 6.3-12 V
Filament Current: 300-150 mA
Plate Voltage (max): 330 V
Plate Current (max): 6 mA
Max Plate Dissipation: 1.1W
- Plate 2
- Grid 2
- Cathode 2
- Filament 2
- Filament 1
- Plate 1
- Grid 1
- Cathode 1
- Shared Filament
Different 12AX7 Names And Labels
12AX7 Vs The ECC83
If you have ever opened up your amplifier expecting to find a 12AX7 but found some ECC83s instead fear not!
There is actually no difference between the tubes except for the name and place of manufacture.
The 12AX7 label is what was used when the tube type was manufactured in North America.
Today there are no factories in North America making tubes anymore but by request manufacturers will label them as such. After all, the design is the same.
The ECC83 on the other hand is the label that was applied to the tube type by European manufacturers yet it is the same tube.
So if you ever find an ECC83, you can replace it with a 12AX7 and vice versa.
They are the same tube type with just a different nomenclature so go ahead and use that ECC83 in place of a 12AX7!
7025 Pre-Amp Tube
The 7025 is a name that was given to an upgraded version of the 12AX7 in the 1960s. The upgraded version was quieter and more robust than the standard 12AX7 of that time.
They were less microphonic and just an all around better tube.
A few years later, manufacturers of the 12AX7 began to make their tubes in the same manner as the 7025.
The new design became standard and the 12AX7 was a better production tube.
Today some manufacturers still claim to make a version of the 7025 with better metals for a more robust tube.
They are a bit more expensive but are quieter than the standard 12AX7.
These quieter version of the 12AX7 are only beneficial when used as the input tube in a preamp.
The noise floor is very low at the input tube and so the quieter the input tube the better.
5751 Pre-Amp Tube
The 5751 version of the 12AX7 was created as a military spec tube. This tube was given extra support internally and had matched triode sections.
The amount of gain produced by a 5751 is also lower than the 12AX7.
Your standard 12AX7 has a gain factor of 100 where as the 5751 is 70.
Some people refer to the 5751 as being a 12AT7 because it too has a gain factor of 70. But this simply is not the case as the plate resistance of a 5751 is the same as a 12AX7.
You can use a 5751 in place of a 12AX7 but keep in mind it will produce lower gain.
So if you want to tame an amplifier with too much gain, this is the tube you want to use in select spots of a pre amp.
You can also use this tube in the position of the phase inverter to produce less power from an amplifier.
12AX7-A And 12AX7-B
The 12AX7-A and the 12AX7-B are two tubes that are identical in nearly every way.
You may have come across these two labels in your hunt for tubes and were curious about the differences.
They are made in the same batches and are the same tube, but some of them come out producing high gain.
The 12AX7-A is a higher gain tube than its counterpart the 12AX7-B. For all intent and purpose, the 12AX7-B is simply a screened tube selected for its high gain and low noise.
The ones that had higher gain and were noisier may have simply been named 12AX7-A.
Mesa Boogie and Peavey use 12AX7-A labelled tubes because of the higher gain, but this comes at a price of a noisier tube sometimes.
But if you want a gain monster for an amplifier and are not worried about noise, the 12AX7-A is your animal.
If you want high gain and a quieter, even softer tube sound, the 12AX7-B is your best bet.
Microphonics And Noise
Checking a 12AX7 For Microphonics
One of the common terms used among tube amp owners is microphonic. This is a go to term used when issues with an amp arise.
But what does it really mean when a tube goes microphonic?
When tubes are manufactured, they are tested for a number of different things to verify they are a good tube.
Noises are one of those tests, and in particular, whether or not the tube is microphonic.
A microphonic tube is a tube that is susceptible to external noise and amplifies it. In most cases the tube will ring like a bell if tapped on.
Now, most tubes will make noise when they are used in the input position. So it’s important to note that it will make a sound when tapped on because of the noise floor.
It’s just that a rugged tube with low microphonics will normally make a thud type sound. A microphonic tube on the other hand will ring and produce very high frequency noise.
Just a word of caution, when you tap on a tube to check for microphonics, don’t tap too hard. The tube is an electromechanical device that can be damaged if it is hit too hard.
Are Microphonic Tubes Bad?
It’s a fact that tubes produced today are not what they used to be.
In some cases there are little to no tests done by some manufacturers. Most tests are done by amplifier builders to weed out any bad tubes.
Sometimes batches of very microphonic tubes will make their way to an amplifier manufacturer, but this does not mean that microphonic tubes are all bad tubes.
Infact, some microphonic tubes have the potential to outlast a less microphonic tube. And all tubes are microphonic to some degree so it’s impossible to have a perfect tube.
The trick with above average microphonic tubes is making sure to prevent using them in a position with a low noise floor.
For example, you do not want to use a tube that is microphonic at the input position of a pre-amp or a tube driven effects loop.
The noise floor at these positions is very low and the ring of a microphonic tube will be very noticeable here.
It is always better to use a microphonic tube in a position where the signal levels have already been amplified and are larger.
Some tubes are more microphonic than others however, so it’s important to keep that in mind.
If the tube is so microphonic that it is unstable, its best not to use it at all. If its got a slight ring to it but is somewhat stable, it will probably be ok.
Understanding 12AX7 Tube Noise
So now that we have touched on microphonics, it’s important to talk about tube noise.
You see, tube noise is actually more of an issue than microphonics. A noisy tube will make crackling noises, hum and even pop occasionally.
Noises that are not appreciated, especially in an amplifier designed for high gain guitar or a Hi-Fi stereo system!
But all tubes have a noise floor, this is not avoidable.
Typically a tube will produce a soft hiss or white noise as it is often referred to. This is normal and will be different from tube to tube as some produce more white noise than others.
You are more likely to notice white noise in a guitar amplifier built for high gain.
Amplifiers like this have many triode stages connected in series. Each stage will amplify the white noise of the input stage making it more noticeable.
And so using the quietest tube you can find in the input or first stage will always be your best bet.
Some amplifier companies like Mesa Boogie take the time to find the quietest tubes within their batches.
They then sell them for the first stage or input stage of high gain amplifiers. So if you play with high levels of gain, you are more likely to have a better experience with these quieter tubes.
They will produce less white noise and are perfect for the first stage in all amplifiers!
Crackles And Pops
So what kind of noise does a tube have to make before it should be replaced? Well if your 12AX7 begins to make any pops or crackles, it is probably time to replace it.
Now, if the tubes are brand new and you put them in and they crackle. Don’t worry, in most cases this is just the tube burning in and cleaning impurities off of its internal parts.
We recommend that you leave the amp on for a few hours to let the tube settle in.
If after a few days it continues to make these sounds, it is probably a bad tube.
But if you have had the tubes in your amp for years and it starts crackling or popping. It’s time for a replacement.
12AX7 Comparison And Reviews
While the 12AX7 tubes are designed around certain gain parameters etc., the sounds are different.
Each manufacturer produces a 12AX7 that is their own and each one has strengths and weaknesses.
Some are darker sounding while others are bright. Some are noisy while others are not.
So we have created a comparison chart with some reviews on what we feel these brands of 12AX7 brings to the table.
We chose these tubes because we feel they are some of the best 12AX7/ECC83 tubes on the market that are still affordable and easy to get.
The 3 main rows below are microphonics, over all noise and frequency response. These are what we feel are some of the main things you would want out of a 12AX7.
So we have given each tube a rating for each of its strengths and weaknesses.
- Microphonic Rating ( 1 being very low and 5 being very microphonic)
- Noise Rating ( 1 being very low and 5 being very noisy)
- Frequency Response (1 being very dull and 10 being very rich and lively)
(Clicking on a picture will take you to Amazon for more information)
The JJ ECC83 is a very popular 12AX7 among guitar players and musicians. This tube is very well balanced and is relatively low in noise.
This tube is warmer sounding than other 12AX7 and is great if your looking for a darker sound. If you have a harsh or bright sound, this tube will tame that harshness.
The Tung-Sol 12AX7 is another popular tube. This tube is very musical and rich in harmonics. The highs are smooth rather than harsh yet very pronounced.
The tube has the ability to tame high end harshness but isn’t dark or muddy. Good sounding tube with great drive.
The Genalex Gold Lion 12AX7 is very smooth. This tube has a very broad mid range that is pleasing to the ear.
The highs and lows are well balanced giving this tube a very full dynamic sound. This is considered the best 12AX7 in production and the change is noticeable!
The Mullard 12AX7 is a great tube for Hi-Fi use.
This tube has a flat response and is a very smooth sounding tube. It is not an optimal choice for guitar amps as it doesn’t quite color the sound like other tubes we have tried.
But for Hi-Fi use, this is a fantastic 12AX7.
The Electro-Harmonix 12AX7EH is another great choice. Its a well balanced but smooth full bodied tube.
Many amplifier companies load their amplifiers with these tubes in the factory because of their rich sound.
The Groove Tube 12AX7C is a rebranded Shuguang 12AX7A. These tubes are smooth with a brighter sound.
This tube has that Marshall bite yet has a full bodied mid-range and solid low end.This is a great tube for rock and metal players.
The Tube Amp Doctor ECC83WA is a harmonically rich tube with a thick mid range and great low end.
It has a bright top end that is similar to a 12AX7-B but softer to a degree. Over all its a great tube made by Shuguang and designed by TAD.
Which 12AX7 Sounds Best?
This depends on the sound you are going for. If your looking to open up your sound giving it blossom and a very dynamic sound, the Genalex Gold Lions will get you there.
They are an incredible tube and will work in most circumstances.
If you are looking for a darker sound, the JJECC83 is pretty dark to the point of being almost muddy in some amps.
If that isn’t quite right for you, then the Tungsol would be next in line. it isn’t quite as dark, but is a pretty balanced tube with great projection. Less congested than the JJ.
For those looking for a tube with great high gain bite that is less expensive then you may want to consider one of the Shuguang tubes.
Over all, the Tungsol would probably be the best bet for bang for your buck in most cases. but if price is not an issue, you will want to try the Gold Lions.
How Long Do 12AX7 Tubes Last?
A 12AX7 is generally designed to provide 10,000 hours of service before you should considering changing it.
However, tubes are like light bulbs and can just stop working one day out of the blue. Even under the 10,000 hour mark.
A 12AX7 is an electro-mechanical device that also contains a heater element. Shock to the tube could displace the elements inside causing issues.
Even the heater element could just burn out. Nothing is guaranteed!
One tube may give you the full 10,000 hours while another only give you 4.
This is the way it goes with tubes. However most manufacturers provide great warranties to make sure you get good tubes.
So always buy from a good vendor. This is why we recommend Amazon.
My 12AX7 Flashes At Start-Up
A flash from a 12AX7 is normal at start up and is nothing to worry about. Over the course of time this may even go away.
If however you see your power tubes flash at start-up, this is not normal! If this happens, turn your amp off and get it serviced.
Flashing power tubes is bad and could cause damage to the amp.