The invention relates to a vapour generator for the deposition of a metal coating onto a substrate (7), preferably a steel strip, that comprises a vacuum chamber (6) in the form of a housing including a vapour deposition head or ejector (3) in tight communication via a supply duct (4) with at least one crucible (1) containing the coating metal in a liquid form and located outside the vacuum chamber (6), characterised in that the ejector (3) includes a longitudinal slot for the vapour outlet acting as a sonic throat and extending on the entire width of the substrate (7), a filtration medium or a charge loss member (3A) made of a sintered material being provided in the ejector immediately before said slot on the vapour path in order to equalise the flow speed of the vapour exiting the ejector (3) through the sonic throat.