12AX7 / ECC83 / 7025

The 12AX7 is by far the most common out of all preamp tubes. Also known as the ECC83 or the 7025, it is a dual triode tube that has the highest voltage gain of all preamp tubes, with a mu of 100. Widely used across guitar amplifiers and hi-fi applications.

For less gain, you can replace a 12AX7 with 5751, 12AT7, or a 12AY7