A pressure multiplying apparatus for supplying, at a regulated pressure, a control fluid in a closed system to control a device such as a hydraulically operated valve. The system is used to control either 1; automatic blowout control devices around a drilling well or 2; to use such control equipment to control the pressure on the drill string used to drill the well. Such system includes a pump for supplying the control liquid to the valve being controlled at a pressure which is a constant factor greater than the pressure of the gas used to power the pump. Valve means are provided to bleed the pressure of the control fluid proportionally to the lowering of the pressure of the power gas. This includes a differential piston exposed on one side to the pressure of the power air and on the other side to the pressure of the control fluid from the pump. The two areas of the differential piston have approximately the same ratio as the multiplying pressure factor of the pump. Means are further provided so that a constant drill pipe pressure can be maintained by using the pressure of the fluid in the drill pipe to regulate the control fluid.