I´ve been reading arround and many peoples say that the change can be done, in a expert page it´s said that the change can be done but that it doesn´t allays work.
Substitutions:
* means appropriate for parallel filament circuits
# means may not work in all circuits
Preamp and driver tube substitutions:
12AX7 (high gain dual triodes with pinout 9A)
12AD7* 12DT7 7729
12AU7# 5751* B339
12AU7A# 5751WA* B759
12AX7 6057 CV4004
12AX7A 6681 E83CC
12AX7WA 6L13 ECC803
12BZ7* 7025 ECC83
12DF7 7025A M8137
12DM7* 7494