I am trying to do a trill over the break between the Clarion and Chalumeau registers, between A and B.
You are meant to add the upper side key to the A, see e.g. here.
This is what the top two side keys are for. From a throat A, adding the top side key results in a B.
The B does not have a very full sound. Is there a way to add more to it using the embouchure or something? Or does it just sound a bit weaker than trying to trill the usual way?