Be aware that the 12AT7, 12AU7, and 12AX7 are distinctly different tubes with different impedances and gains.

There are ignorant people who seem to think they can play around with interchanging them, and this is not smart at all.

You should NEVER replace one of these tubes with one of the others, because you are putting it into a circuit that is not designed for it and distortion will result in most cases.

As someone who designed circuits with these tubes for a number of years, I know what I am talking about.

They each have a very different transconductance and require very different plate currents to operate properly.

They are not interchangeable.