Hi. I have a behringer AC 108 vintager amp. its an amp with a tube in the preamp.
this tube is originally a 12ax7 but I read some reviews that said that if I replaced the
tube with a 12 au7 I would get better control of the gain.

The question is: Is this a straight swap over? no adjustments or stuff like that to be done?

This is a preamp tube. Just pull out the 12AX7 and plug in a 12AU7. Don't expect much though. It is a behringer and it is a hybrid amp. The tube doesn't contribute much.
that's good.
I actually like the amp very much. not expecting to much is something I do with everything I buy. things are much better that way
of course it's not the best but for the price its alright enough for rehearsal.
thnx for the quick answer.