22 Different Brands of 12AX7 Tubes Tested Against Each Other

Guest writer Kurt Prange rates 22 12AX7 valves   22-Mar-13

22 Different Brands of 12AX7 Tubes Tested Against Each Other


DIY guitar enthusiast and Amped guest blogger Kurt Prange, who works at Amplified Parts in Arizona, compares 22 different brands of 12AX7 tubes to see how they differ...

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.


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 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.

Go to the next page to see which brand of 12AX7s rated the highest.

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4 Comments...  Post a comment    original story
Willian    Said...

Discovered your site using Ask. This is a pretty good aitrcle. I hope to see you remove the main point of this aitrcle and make another second aitrcle, maybe include a vid, too?

11-Apr-13 01:48 AM


Willian    Said...

Discovered your site using Ask. This is a pretty good aitrcle. I hope to see you remove the main point of this aitrcle and make another second aitrcle, maybe include a vid, too?

11-Apr-13 01:48 AM


Myles Rose    Said...

I would love to know how they measured "gain" as there is some very misleading information here. Mu can be very high when the actual gain is very low or a tube can have lower conductance or Mu with high gain. One needs to know what the plate resistance of the tubes are and publish or supply that data. This can be a common case of somebody with a tube tester and a bit of knowledge making some incorrect assumptions. I would also love to have more data on his NOS tubes that did not fare well.

The good info is showing how inconsistent things are.

12-Apr-13 05:10 PM


Jim    Said...

Interesting however the Mu voltage is not relative to being any ruler if the tone is horrible ! Metal guys tube roll a few sino ax7 as they grind early and chewing metal is there if you want that ! I want Blues-pop-classic rock with complex thick breakup . Chords have to work also. The Phase inverter is a big deal for me. The sovtek 12ax7Lps keeps it together best yet, Sylvania was good and deferent.

Listen carefully , my smearing sounding highs was in my case from the FX loop tube too dang bright Amperex 12ax7 ! A original Tungsol used did the magic for me. I want tubes that have that tube sound , Sovtek 12ax7wa wb and wc have those round notes. Get your hands on a good Phillips 12ax7 long plate for OD socket its badazz .

Voltage gain measurements are cool and worthy effort with 6 tubes powered at once. What we think we want is not a voltage gain number for me.

Know your amp what tube affects what then common sence testing is inevitable. The v1 is your best sounding usually then the EQ recovery 12ax7 on too your phase inverter. Unplug your effects loop some where and listen too the highs as mine changed the HIGHS way BAD when plugged in. Hope you have fun because it is exploring and adjusting even how you play is affected. Amazing fun !

17-Dec-14 02:36 AM


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