by EWBrown » Tue Oct 21, 2008 9:17 am
5963s and 6189s are also very good 12AU7 substitutes. These are the industrial or computer tubes, and seem to be built to a higher standard, but generally sell for less than the "recognized" 12AU7 / ECC82s.
12AU7s were originally intended as RF low power tubes, as were their "half-siblings", the 6C4. They work well for audio, but there are better choices.
If you don't mind re-wiring the filament connections (use pins 4 and 5 only) , then 6CG7 / 6FQ7 is a good choice, it has the character of a 6SN7, but in a 9 pin miniature envelope.
As I mentionmed in the other related posting, the in PI section, a 12AX7 will tend to draw excessive (for 12AX7 PD ratings) current, and may require that the 33K and 27K resistors be changed to higher values, perhaps 56K and 47K would be more suitable.
/ed B
Real Radios Glow in the Dark