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12AX7 Comparison of Current Made Tubes
All 12AX7s have an amplification factor (μ) of 100 ("high-mu"). Ideally, every 12AX7 should yield the same gain when placed in the same circuit. However, in reality there are comparative gain differences among 12AX7s.
- This graph is based on the average gain and noise measured from different tube brands using the same number of samples for each brand. It should not be assumed that every single 12AX7 from a specific brand will match its place on this graph.
- The gain rating is only a relative reference (e.g. a rating of "10" does not mean ten times louder than a "1").
|Part Number||Gain Rating (1 - 10)||Drawing||Glass Dia.||Noise|
|Frequency Response (Seperated into 3 Ranges||Drawn to Scale||1 = Low Noise |
4 = High Noise
|(.870 - .878")|
|This Russian made Mullard tube is powerful and has the most gain of all the tubes in our comparison. It offers easy access to break-up with lots of top-end harmonic texture.|
|(.870 - .882")|
|This Russian made Tung-Sol tube is powerful, tight, punchy and articulate.|
|(.840 - .850")|
|This long plate gold pin Slovakian tube is very similar to the plain-pin ECC803 version, but with slightly more gain and top-end harmonic texture.|
|(.856 - .875")|
|This Chinese tube has a tight mid frequency focus.|
|(.869 - .877")|
|This Russian tube offers more gain, bite and slightly easier access to break-up than the 12AX7WA version.|
|(.840 - .853")|
|This gold pin Slovakian tube takes the characteristics of the plain-pin 12AX7/ECC83 version and gives them more gain.|
|(.870 - .878")|
|This Russian tube has a lot of bite with easy access to break-up.|
|(.843 - .854")|
|This Slovakian tube has a similar sound and feel to the plain-pin ECC803 with slightly more low end emphasis.|
|(.842 - .854")|
|This long plate Slovakian tube has tight mid frequencies and top-end chime.|
|(.859 - .872")|
|This Chinese tube from Tube Amp Doctor sounds very similar to the 7025WA version. It measured in with slightly higher gain levels, but the difference seems insignificant to the ears.|
|(.871 - .879")|
|This long plate Russian tube is smooth and tight. It has warm lows and mids with a top-end that offers more presence than the 12AX7WA.|
|(.851 - .867")|
|This is Tube Amp Doctor's offering of the commonly re-branded Chinese tube.|
|(.858 - .879")|
|This is Ruby's offering of the commonly re-branded Chinese tube.|
|(.857 - .869")|
|This is the "B" version of the commonly re-branded Chinese tube straight from the source. It sounds very similar, if not identical, to the original.|
|(.879 - .883")|
|This Russian tube is full and tight with emphasis on the lows. (The glass diameter can be on the fat side which may be a tough fit for some tube shield bases).|
|(.870 - .873")|
|This long plate Russian tube sounds like a slightly lower gain version of the 12AX7LPS.|
|(.849 - .873")|
|This Chinese tube from Tube Amp Doctor offers warmth with good presence and articulation.|
|(.855 - .866")|
|This is GrooveTubes' offering of the commonly re-branded Chinese tube.|
|(.862 - .873")|
|This is the commonly re-branded Chinese tube straight from the source.|
|(.809 - .814")|
|These NOS tubes are becoming scarce. This drawing is of an RCA circa 1970. We used RCA, GE and Sylvania brand tubes for our comparison.|
|(.840 - .853")|
|This Slovakian tube is smooth and warm. It offers easy access to break-up with lots of top-end harmonic sparkle.|
|(.870 - .876")|
|This Russian tube is smooth and tight. It offers warm lows and mids while staying conservative on the top-end texture.|
A Comparison of Current Production 12AX7 Tubes
RCA and Sylvania first introduced the 12AX7 to the world in the late 1940's. Today, it is by far the most common preamp tube used for electric guitar amplification and there is an almost overwhelming supply of current production options to choose from. The purpose of this comparison is simply to provide guitar players with a frame of reference to help in finding the best current production 12AX7 for their needs.
Also included in this comparison are direct substitutes for the 12AX7 including: ECC83, ECC803, 7025 and 5751.
The 12AX7 is a 9-pin miniature, high-mu, twin triode. What does that mean? The 9-pin miniature part tells us what kind of socket the tube plugs into. It has nine thin pins arranged along the circumference of a circle with a diameter of just under half of an inch. There is a large gap between pins 1 and 9 to ensure that the tube can only be plugged in one way. The high-mu part is referring to the electronic specification called "amplification factor," abbreviated by the Greek letter μ (mu - pronounced 'myoo'). This gives us a general category of amplification that the tube was designed for as compared to other preamp tube types like the "medium-mu" 12AU7. The twin triode part tells us that there are two identical and independent triodes contained within the one tube. A triode is the most basic tube structure that allows for signal amplification and it has three electrodes: plate, grid and cathode.
The Gain Test Setup
Each tube sample was tested in the same (Fender black face style) preamp circuit as shown in the schematic below with bass and treble set to their maximum. A frequency generator was used to measure voltage gain and plot the frequency response from 10 Hz to 18,000 Hz. From the frequency plot it could be seen that there are three extrema at about 50 Hz, 400 Hz and 4,000 Hz. These three frequencies were then used to quickly measure the low, mid and high frequency voltage gain on the same number of samples for each tube in our comparison. The average from each tube's samples was then calculated and used for comparison. The smallest average low frequency gain tube was used as the reference for a gain of "1" and the largest average low frequency gain tube was used as the reference for a gain of "10". This gain reference method was then repeated for both the mid and high frequency averages.
The Noise Test Setup
Each tube sample was again tested in the same preamp circuit and an oscilloscope was connected to the amplifier output to monitor the amplitude of the voltage spike produced by tapping on the tube's glass envelope. The average voltage spike from each tube's samples was then calculated and used for comparison.
The Listening Test setup
A custom 12AX7 switching box was constructed to allow for a quick switch comparison of six tubes at a time while plugged into the V1 socket of a Marshall JCM800 2203 guitar amplifier. A separate filament transformer was used on the switching box to simultaneously heat all six comparison tube filaments without overheating the amplifier's power transformer. A shielded cable was constructed to plug into the amplifier's V1 socket and transfer its pin 1, 2, 3, 6, 7 and 8 connections to one tube socket at a time in the switching box. Listening tests were then conducted while playing guitar and switching between the tubes at various amplifier control settings to come up with tonal descriptions for each tube.
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