I would contact Kara Chaffee at deHavilland Hi Fi. She has forgotten more about tubes then most people know.
dehavillandhifi.com
or:
kara@tubewizard.com
dehavillandhifi.com
or:
kara@tubewizard.com
pre amp tubes
Are 12AX7 and 12AU7 tubes interchangeable in preamps and if so what are the differences. Any suggestions for not overly expensive tubes? I have used RCA nos and Tungram before but can I do better?