The amplifier includes a housing 2, 3 containing a chamber 4 provided with a delivery outlet 8 containing a non-return delivery valve 10. An inlet pipe 7 projects into and terminates within the chamber 4 and a resilient D-section obturator ring 13 is engaged with and located about the pipe to be resiliently-movable in the chamber. An annular exhaust aperture 12 surrounding the pipe can be sealed by the obturator ring 13, the obturator ring being responsive to fluid flow in the inlet pipe 7 such that fluid flow causes the obturator ring to oscillate between conditions which alternately permit and prevent fluid from leaving the chamber through the exhaust aperture 12 thereby causing a pulsed pressure increase in the fluid flowing through the delivery outlet. An adjuster 23 is provided for adjusting the distance by which the fluid inlet pipe 7 projects into the chamber and thus vary the distance between the obturator ring 13 and the annular exhaust aperture 12. The adjuster 23 is rotatably engaged with the pipe 7 and with the housing and moves the pipe using co-operably inclined faces. The obturator ring 13 and the exhaust aperture 12 are preferably shaped to create a venturi effect as fluid flows past them.
