22 Different Brands of 12AX7 Tubes Tested Against Each Other

Posted on Feb 6, 2014

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.

22 Different Brands of 12AX7 Tubes Tested Against Each Other
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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. 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...

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