@Quantensalat To me that circuit looks like the transformer just forms a series tank with the bunch of caps. Looking at the values and the transformer on the photo that could work out to the tatget frequency range of this amp. At resonance, the BJTs feed current into the tank, but the tank’s circulating current is not necessarily in phase with the BJTs driving it. Note that this is not a bridge topology, and looks a lot like a resonant converter (a DC/DC of DC/AC topology).