Confused about which tubes to get?

The differences between 12AX7, 12AT7 , and 12AU7 tubes
(Here are the most common applications and options for substituting tubes)

Tubes, or valves, are used in all sorts of of electronic equipment. Commonly, you will find the in guitar amplifiers, expensive vintage preamps, HAM radios, microphones, and they come in thousands of different brands, models, and constructions. While there are hundreds of different types of tubes, only a select few are used in pro audio equipment.

This handful of frequently used tubes, 12AXY, 12AT7, and 12AU7, can still be confusing to differentiate and understand which is used for what purpose and whether one can be substituted for another.  Below we will try to clarify the most common differences between the 12AXY, 12AT7, and 12AU7, and in what circumstances they can be interchangeable. 

MAIN DIFFERENCES:  12AX7 VS 12AT7 VS 12AU7

The most noticeable difference between the 12AXY/12AT7/12AU7 tubes is the amount of gain, or power, they provide. The 12AX7 has the most gain factor with 100, the 12AT7 comes in next with 60, then the 12AU7 with the lowest gain at 20. Gain isn't necessarily proportionally to sound quality, in the contrary, lower gain values allow for high headroom, meaning a warmer and cleaner sound. This being said, one should should a 12AX7/12AT7/12AU7 depending on what application the tube will be used for and the type of sound wished-for.

12AU7XXX