The 5751 is considered a substitute for the 12AX7 not the 12AT7.
Here is what tube dealer Brent Jessee says about it:
5751: This version was usually the one made for the US military. These have a lower gain than the 12AX7, but are otherwise totally compatible. Great hi-fi tubes, as the lower gain reduces noise and microphonics.
I’d be interested to know what atmasphere (Ralph) thinks of this opinion. Ralph knows more about tubes and their uses than the rest of us combined. He has been designing and building tube amps and preamps for quite a while.
As Ralph said, you need to be careful about substituting tubes. There may be consequences, like damaged gear, beyond the immediate change in sound.