The heater current of a 5751 is slightly higher than a 12AX7, be sure that your power supply can take it before making this change willy nilly.
Additionally, the gain is 70 vs. 100 for a 12AX7, so your signal to noise ratio may improve, but the lower gain may be a compromise in certain systems.
12AU7 and 12AT7 have no place being substituted for a 12AX7 as they are electrically dissimilar. However if you are more competent than the designer of your circuit, by all means have at it. You will do no damage.
http://bitsavers.trailing-edge.com/components/rca/1961_RC-21_RCA_Receiving_Tube_Manual.pdf
And if your circuit is a hybrid tube/transistor Chinese designed circuit based on op amps, you may very well be more competent than the designer of the circuit.
Additionally, the gain is 70 vs. 100 for a 12AX7, so your signal to noise ratio may improve, but the lower gain may be a compromise in certain systems.
12AU7 and 12AT7 have no place being substituted for a 12AX7 as they are electrically dissimilar. However if you are more competent than the designer of your circuit, by all means have at it. You will do no damage.
http://bitsavers.trailing-edge.com/components/rca/1961_RC-21_RCA_Receiving_Tube_Manual.pdf
And if your circuit is a hybrid tube/transistor Chinese designed circuit based on op amps, you may very well be more competent than the designer of the circuit.