PPS The post with the AU7 cathodyne is from Porky, who may no longer
be visible to some.

Porky, the extra drive is presumably because of the voltage
amplifier section of the AX7, not the section used for the
concertina, which has no gain itself no matter what valve you use.

If you need more gain, you may find it better to create it earlier
in the signal path.

cheers, Ian