The two tubes are based on the same tooling. A 12AU7s is exactly half the plate structure of a 6SN7. They have the same gain and bias in a similar fashion.
It is also double the filament current. It is probably that fact that might stop you trying something like this. So you should check with the manufacturer of the amp to see if there is any extra filament current capacity. With 4 6SN7s you will need 1.2 amps extra.
It is also double the filament current. It is probably that fact that might stop you trying something like this. So you should check with the manufacturer of the amp to see if there is any extra filament current capacity. With 4 6SN7s you will need 1.2 amps extra.