The amplifier includes a housing containing a chamber with a delivery outlet containing a non-return delivery valve. An inlet pipe projects into the chamber and a resilient obturator ring is engaged with and located about the pipe to be resiliently-movable in the chamber. An annular exhaust aperture surrounding the pipe can be sealed by the obturator ring, the obturator ring being responsive to fluid flow in the inlet pipe 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 thereby causing a pulsed pressure increase in the fluid flowing through the delivery outlet. An adjuster is provided for adjusting the distance by which the fluid inlet pipe projects into the chamber and moves the pipe using co-operably inclined faces. The obturator ring and the delivery valve are shaped to create a venturi effect.