An exemplary control stage of a steam turbine is disclosed, which includes plural staging valves circumferentially distributed around the turbine for regulating steam admission flow so as to control the loading of the turbine. Nozzle chambers are connected to a downstream end of each staging valve. An arc of admission forms the downstream portion of each nozzle chamber, and control stage nozzles in the arcs of admission define the downstream end of the nozzle chamber. Each nozzle chamber has at least two arcs of admission, each with a different circumferential dimension.