by Ty_Bower » Sun Feb 17, 2008 8:05 pm
There are many alternate tubes which can be tried, many of which are virtually drop-in replacements. Instead of the 12AX7, you could try an ECC83, ECC803, 7025, or 5751. All are pin compatible, and should have similar characteristics to the 12AX7 (except the 5751, which has less gain). Instead of the 12AU7, you could try an ECC82, ECC802, 5814, or 12BH7.
The 5751 / 12BH7 appears to be a popular combination.