Abstract: Automatic starting of a hydrokinetic amplifier 10 is accomplished by: an overflow pilot valve 32 responsive to output pressure for opening an overflow line 30 when output pressure drops below a predetermined level; and a start-up pilot valve 36 responsive to reduced pressure within amplifier 10 upon opening of overflow pilot valve 32 so that the reduced internal pressure opens the start-up pilot valve and admits vapor into the amplifier to merge with the already flowing liquid and start up the amplifier. Preferably a safety pilot valve 38 arranged in the vapor input line 13, in series with start-up pilot valve 36, closes the vapor input line whenever liquid input pressure to the amplifier falls below an adequate level.