Good to shear off some gain and some other uses.
The 5751 is a tube I always keep on hand. Some modern amp makers have been known to swap ECC81/12AT7s in the phase inverter stages for 12AX7s as a "design change". If you have an amp that has 12AT7 marked by a socket and a 12AX7 in the socket & a 12AT7 won't work in that position this is a great option. Some Dr Z amps were like this. As a disclaimer I must say I have no idea if they came from Dr Z this way or not for sure, but this tube was an easy fix. Compared to an other Dr Z of the same model the sound was nearly identical, and the amp had been restored to what the customer wanted. The 12AT7 would not work due to plate resistance, but a 5751 has identical plate resistance to a 12AX7/7025.