Abstract: Apparatus for and a method of creating positive pressure pulses in a stream of drilling mud being circulated through a drill string is disclosed. The apparatus includes a restriction in the drill string, a valve member located below the restriction, a cylinder located in the drill string above the restriction, a valve stem connected to the valve member and extending upwardly through the restriction into the cylinder, a piston located in the cylinder and connected to the valve stem, and a restricted passage through the piston through which drilling mud can flow and equalize the pressure across the piston. In operation, opening a pilot valve allows the fluid above the piston to flow through the piston rod to the drill string below the restriction. This creates a pressure differential across the piston that moves it upwardly and causing the valve member to further restrict flow through the valve seat. This creates a pressure pulse.