Helped tame my Blackstar HT-5RH
My Blackstar HT-5RH went from no gain to distortion with barely a turn of the knob. Not so good for getting that classic rock crunch. So I replaced the stock 12AX7 preamp tube with the JJ 5751 and it made it so I have from a light crunch to heavier crunch in between 7-9 o'clock for more of that classic rock feel while retaining the tone and character of the amp. It may have lost some treble after 12 o'clock, but by that point you still have all the gain you could need.