a little 5751 info
Unlike a 12AX7, the 5751 has a lower gain of 70 (100 for a 12AX7) just like the originals. The tube is lower impedance, and has predominent 2nd harmonics for a sweet, smooth tone. The 5751 is famous for its linear response and good transconductance and this tube holds true to that specification.
The new tube also has the cathode constant of the original and will work wonders in Marshalls.
This means the tube will not be destroyed by the direct coupled cathode follower found in many amps, and a problem with all 12AX7's that many do not know about.
The 5751 has a bit less gain than a 12AX7, (30% less) and a more linear tone, Smoother, with outstanding clarity and "chime." Overdrives in a different manner when compared to other tubes (not as harsh) Very cool.
Stevie Ray Vaughan preferred them in his Super Reverb and Vibroverb in tube #2 (reverb preamp) for a clean tone with far more headroom than a 12AX7. It is one of the Characteristics of the "chime" tone.
The 5751 is not a 12AX7, it is a different tube with different character.