This 12AX7 is a dual triode currenty produced in Russia. It is used by Fender, Ampeg, Carvin, and Hartke, among others.
The OEM workhorse tube used by major amp manufacturers, with warm round tone. Very low in microphonics, and is excellent for use in guitar amps. You can hit it with a stick and it'll barely bat an eye. This is very important for combo amps, or for high volume application.