You may find that NOS tubes will not last long in the AI pre.JFYI.
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Here is a recommendation from Audible Illusions for a good replacement tube for their preamps:
"6H23EB RUSSIAN MILITARY, RUSSIA - GREY PLATES
Most are NOS currently imported from our sources in Europe. Similar to 6922 Sovtec "Grey Plate" tubes. Closely resemble tubes installed in Modulus, series 2 and 3 preamplifiers between 1988 & 1996. Excellent sonics and reliability."
I have read other threads where this tube is recommended for AI preamps because of its durability. Below is another excerpt from a post about tubes for the ARC SP10 preamp which is also very hard on tubes:
"With the ARC SP10, the first principle of retubing is unfortunately not the sonics, but the survivability of the tubes. Part of the decision -what won't work) on tubes can be determined by looking at the ratings for the various 6DJ8s:
ECC88 = 6DJ8 130V / 1,8W
E88CC= 6922 220V / 1,8W
E188CC= 7308 250V / 2W
6N23P/ Sovtek 6922 300V / 1,8W
6N23P-EV/ EH 6922 300V / 2W
In short, I have had or heard extensive evidence from others that the best sounding tubes for the SP10 are 6DJ8s, but the life of these can be measured in 10's of hours, the usual 6922s: Brimar, Tesla, modern Ei, even the very rugged NOS Siemens E88CC can last only in the 100's.
The only dependable long survivors will be the three at the bottom of the list- the 7308/E188CC and the Russian 6N23P and 6N23P-EB. The Russian 6N23P-EB is like an uprated E88CC/6922 and those and the new EH (haven't tried it) have current ratings above even the 7308."
Sorry, but I don't have the original posters name. 6H23P and 6N23P are Russian tube types and are interchangeable, I believe. Hope this helps.