To build on what Fleschler stated, the ECC83/12AX7 has a gain of 100 while the 5751 tube has a gain of 70 which is a decrease of 30%. This means that while the 5751 will probably be less strident and possibly more transparent, your power amp will have to work a bit harder to get the same loudness level as the ECC83/12AX7. Many guitarists substitute a 5751 for a 12AX7 in their amps because they like the mellower sound it produces. Moving along, the ECC82/12AU7 has a gain of about 19 or 20, which means that if you placed the ECC82/12AU7 in the socket that was meant for the ECC83/12AX7, you certainly would hear the difference. By the way, you might want to try Telefunken ECC83/12AX7's, smooth as silk. They are marginally better than the Mullards. Hope this helps.