# 12AX7 Comparison of Current Made Tubes

#### Graph Interpretation

All 12AX7 tubes 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 NumberGain Rating (1 - 10)Drawing
Glass Diameter
Noise
Frequency Response (Separated into 3 Ranges)Drawn to Scale1 = Low Noise
4 = High Noise
Genalex Gold Pin 12AX7
T-12AX7-GOLD-GEN
Gain rated at 10
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Lows
Mids
Highs

(.871 - .880")
This long plate, gold pin Russian made Genalex B759 is loaded with bite. It offers easy access to break-up and crisp highs making it brighter than the Mullard 12AX7.
Mullard 12AX7
T-12AX7-MUL
Gain rated at 7.3
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Lows
Mids
Highs

(.871 - .880")
This long plate Russian made Mullard is powerful and thick, offering easy access to break-up with lots of top-end harmonic texture.
Sovtek Long Plate 12AX7
T-12AX7LPS-SOVT
Gain rated at 7.3
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Lows
Mids
Highs

(.871 - .880")
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 Sovtek WA, WB and WC tubes.
Tung-Sol Gold Pin 12AX7
T-12AX7-GOLD-TUNG
Gain rated at 5.7
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Lows
Mids
Highs

(.871 - .881")
This gold pin Russian made Tung-Sol is similar to the plain pin version, but with a little more top-end shimmer.
Tung-Sol 12AX7
T-12AX7-TUNG
Gain rated at 5.7
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Lows
Mids
Highs

(.872 - .881")
This Russian-made Tung-Sol is tight, punchy and articulate.
Electro-Harmonix 12AX7
T-12AX7EH
Gain rated at 5.7
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Lows
Mids
Highs

(.872 - .881")
This Russian tube has a straightforward tone with bite and easy access to break-up.
Mullard CV4004
T-12AX7-CV4004-MUL
Gain rated at 5.3
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Lows
Mids
Highs

(.872 - .881")
This Russian-made Mullard CV4004 has shorter plates and a slightly leaner tone than the standard version.
Electro-Harmonix Gold Pin 12AX7
T-12AX7-GOLD-EH
Gain rated at 5.3
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Lows
Mids
Highs

(.872 - .881")
This gold pin Russian tube is very similar to the plain pin version.
JJ Gold Pin 12AX7
T-12AX7-GOLD-JJ
Gain rated at 5.3
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Lows
Mids
Highs

(.842 - .854")
This gold pin Slovakian tube takes the characteristics of the plain pin 12AX7/ECC83 version and boosts them up a notch.
Chinese 12AX7
T-12AX7-CHINA
Gain rated at 5.3
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Lows
Mids
Highs

(.853 - .872")
This Chinese tube offers a lot of value and is commonly rebranded by other companies. It has a tight response focused on straightforward mids and highs.
Tube Amp Doctor 12AX7
Gain rated at 5.3
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Lows
Mids
Highs

(.853 - .872")
This is Tube Amp Doctor's premium selected offering of the common Chinese tube.
Groove Tubes 12AX7
T-12AX7-C-GT
Gain rated at 5.3
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Lows
Mids
Highs

(.853 - .872")
This is Groove Tube's offering of the common Chinese tube.
Ruby 12AX7
T-12AX7A-C5
Gain rated at 4.3
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Lows
Mids
Highs

(.853 - .872")
This is Ruby's offering of the common Chinese tube.
Chinese 12AX7B
T-12AX7B-CHINA
Gain rated at 5.3
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Lows
Mids
Highs

(.853 - .872")
This is the B (noise-selected) version of the common Chinese tube.
Genalex 12AX7
T-12AX7-GEN
Gain rated at 5
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Lows
Mids
Highs

(.847 - .869")
This Chinese-made Genalex offers a pleasant top-end chime and a leaner tone overall than the gold pin B759 version.
JJ Gold Pin ECC803
T-ECC803-GLD-JJ
Gain rated at 4.3
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Lows
Mids
Highs

(.845 - .853")
This long plate, gold pin Slovakian tube is very similar to the plain pin ECC803, but with slightly more gain and top-end harmonic texture.
JJ Mid Gain 12AX7
T-12AX7S-MG-JJ
Gain rated at 4.3
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Lows
Mids
Highs

(.842 - .850")
This Slovakian tube is tight with a lot of articulation in the low mids and good presence in the highs.
Sovtek 12AX7WA
T-12AX7WA-SOVT
Gain rated at 4
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Lows
Mids
Highs

(.868 - .878")
This Russian tube is smooth and tight. It offers warm lows and mids without a lot of top-end texture.
Sovtek 12AX7WB
T-12AX7WB-SOVT
Gain rated at 4
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Lows
Mids
Highs

(.868 - .878")
This Russian tube is very similar to the WA version.
Chinese 7025
T-7025-CHINA
Gain rated at 4
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Lows
Mids
Highs

(.860 - .876")
This Chinese tube has a straightforward tone with tight mid frequency focus.
JJ ECC803
T-ECC803-S-JJ
Gain rated at 4
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Lows
Mids
Highs

(.845 - .853")
This long plate Slovakian tube has tight mid frequencies and top-end chime.
JJ 12AX7
T-12AX7-S-JJ
Gain rated at 4
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Lows
Mids
Highs

(.842 - .854")
This Slovakian tube is smooth and warm with lots of top-end harmonic sparkle.
JJ 5751
T-5751-JJ
Gain rated at 3.7
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Lows
Mids
Highs

(.842 - .854")
This Slovakian tube has a similar sound and feel to the plain pin ECC803 with slightly more low end emphasis.
Tube Amp Doctor ECC83WA
Gain rated at 3
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Lows
Mids
Highs

(.857 - .864")
This Chinese tube from Tube Amp Doctor sounds very similar to the 7025WA version.
Tube Amp Doctor 7025WA
Gain rated at 3
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Lows
Mids
Highs

(.863 - .869")
This high grade Chinese tube from Tube Amp Doctor offers warmth with good presence and articulation.
Sovtek 12AX7WC
T-12AX7WC-SOVT
Gain rated at 2.7
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Lows
Mids
Highs

(.868 - .878")
This Russian tube has straightforward lows and mids with top-end shimmer.
Sovtek 5751
T-5751-SOVT
Gain rated at 2.3
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Lows
Mids
Highs

(.871 - .880")
This long plate Russian tube sounds like a slightly lower gain version of the 12AX7LPS.
Tung-Sol 5751
T-5751-TUNG
Gain rated at 1
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Lows
Mids
Highs

(.871 - .880")
This long plate Russian tube has a straightforward tone with mid focused clarity.

#### A Comparison of Current Production 12AX7 Tubes

RCA and Sylvania first introduced the 12AX7 to the world in the late 1940s. 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.

#### 12AX7 Basics

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 $\mu$ (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.

By Kurt Prange (BSEE), Sales Engineer for Antique Electronic Supply - based in Tempe, AZ. Kurt began playing guitar at the age of nine in Kalamazoo, Michigan. He is a guitar DIY'er and tube amplifier designer who enjoys helping other musicians along in the endless pursuit of tone.