I found this thread while searching for clues about hum issues in the PH14. It goes back a number of years and is about the original design by Tim de Paravicini. It talks about substituting 12AU7 tubes in place of 12AX7 that have much higher gain. Since the PH14 is not the same as the original design I would check with the designer of the PH14 before trying this. However, I did find one bit of helpful information on page three post which is a response from the original designer regarding several questions from users of this Phono board.
"Your passion for maintaining the integrity of your original design has convinced me to revisit my decision to substitute a 12AU7 in position three. What I wonder is must all three 12AX7's be matched, or can I mix various types of AX7's? Please suggest the most accurate and musical combination of 12AX7's that you have tried.
His answer to the first question is most enlightening. Think of the possibilities:
No, they do not have to be matched in any way. V2 is selected for noise. V1 for microphony,
V3 not critical. "
Here is a link to the thread>>> https://www.canuckaudiomart.com/forum/viewtopic.php?f=23&t=13669
"Your passion for maintaining the integrity of your original design has convinced me to revisit my decision to substitute a 12AU7 in position three. What I wonder is must all three 12AX7's be matched, or can I mix various types of AX7's? Please suggest the most accurate and musical combination of 12AX7's that you have tried.
His answer to the first question is most enlightening. Think of the possibilities:
No, they do not have to be matched in any way. V2 is selected for noise. V1 for microphony,
V3 not critical. "
Here is a link to the thread>>> https://www.canuckaudiomart.com/forum/viewtopic.php?f=23&t=13669