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Hi guys ,
I got a good batch of NOS 12AX7's on ebay at a very fair price ( Raytheons an Sylvanias ) , but I haven't been able to find a decent sounding 12AU7 yet . Since the PI in this amp is a Cathodyne ( no gain ), can i use a 12AX7 for PI purposes without getting noticeable distortion? .
Thanks in advance .
SUSO

I'm pretty sure a 12AX7 in the V2 position will screw up the current draw and voltages around it. I wouldn't recommend it. I suspect cliffpickle inadvertently did that on his amp, but he never wrote back to confirm or deny it.

If you want to substitute for the 12AU7 try a 12BH7, 12AV7, or 5814 if you can get one.

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