A knob assembly is disclosed for a control unit of a medical apparatus with a front panel. In order to transfer a turning setting movement through the front panel, a feed-through sleeve is rotatably arranged in the front panel and a knob core is movably arranged in the feed-through sleeve. On the control side of the front panel the feed-through sleeve and the knob core are enclosed by a manipulable element which, through application of pressure, can axially move the knob core in the feed-through sleeve. The knob core on the back side of the front panel moves a resilient locking washer from a locking position to an unlocked position so that turning of the manipulable element also will turn the feed-through sleeve. To improve the sealing at the feed-through location, the front panel is provided with a protection plate which is beaded outwards and surrounding the feed-through sleeve, with the manipulable element tightly enclosing the beaded protection.