Abstract: A control device for regulating the capacity and the built-in volume ratio of a helical screw rotor machine comprises at least one additional valve member which is axially aligned with first and second valve members in the screw rotor machine, the additional valve member being movable between the positions in which said additional valve member contacts one or both of said first and second valve members to form an extension of either or both of the first and second valve members. The additional, first and the second valve members are connected to the operating device to permit displacement thereof. This permits varying the length of the second valve member (the slide valve) in one or more steps, which in turn means that it is possible to displace the gap between the first and second valve members without moving the high pressure end of the second valve member (the slide valve).