12AX7 = ECC83. ECC88 are 6DJ8 or 6922 equivalents. The ECC88 et al are VERY different from a 12AX7 in both electrical parameters and in pin configuration. You could damage your gear or the tubes by inserting an ECC88/6DJ8/6922 into a socket set up for a 12AX7. Variants of the ECC83 include the ECC803, 803S, and probably some others using the English naming system adopted by Mullard. (Mullard CV4024 are 12AT7s, for example, but I don't know the 12AX7 equivalent name.) The 5751 is often sub'd for a 12AX7, but it is not electrically identical. The 5751 has a lower "mu", for one thing. This means that you will have a little less gain with it, so if your phono gain is at all marginal, you might find that it is insufficient with a 5751. However, 5751 will "work", and lots of folks seem to like it in circuits designed for 12AX7. Larryi, I disagree that all-in-one preamps are inevitably inferior to separate line and phono stages. In fact there are many inherent advantages to the one-box solution, but you have to pay the big bucks for the best ones, just as with separates.
