12x7 tubes

Any of the old USA - made 12AX7’s (GE, Sylvania, Baldwin ...) are very good choices! I prefer them over any present day production. Old UK Mullards are really over-hyped and way too costly! I like the green ink Baldwins (made for use in the Baldwin organs).

You can also try the 5751 as a lower-gain alternate!

comments in these info sources are generally fairly accurate, based on my own experience

https://www.audioasylum.com/scripts/d.pl?audio/faq/joes-tubes.html

https://www.audiotubes.com/12ax7.htm