An air-cooled transformer (10) is provided. It comprises a core (20), an inner winding (30), an outer winding (40) having an outer boundary face (42) on its outside face, an air gap (50) between the inner winding (30) and the outer winding (40), a bobbin (60) comprising a dielectric material, located adjacent the outer boundary face (42) of the outer winding (40) and supporting the outer winding (40) from its outside face, wherein the bobbin (60) comprises supporting portions (70) which extend into the air gap (50), characterized in that,the bobbin (60) is mounted to the core (20) via at least one fastening element (100) and the fastening element (100) is adapted for positioning the bobbin (60) and the outer winding (40) with respect to the inner winding (30) for adjusting a stray inductance of the transformer(10).