In order to optimally control the air supply in a diverging gap between a card main cylinder and a doffer roll, an air guide plate with a predetermined spacing on the narrowest part between the air guide plate and the surface of the main cylinder and with an aperture angle is arranged close to the main cylinder so that a converging gap is provided between the air guide plate and the surface of the main cylinder. In this converging gap, a pressure head builds up in operation through the air conveyed from the surface of the main cylinder, which leads to an overflow over a nose of the air guide plate. This overflow helps the fleece, which is loosened from the main cylinder, to more easily reach the doffer surface. In order to control the quantity of overflowing air, the gap is to be determined by trial. The air passing through the gap may be taken up from a conduit shaped foot part, which can be connected to a vacuum source, for withdrawing particles of dirt which arrive in the conduit and, and to control the vacuum in the conduit. This pressure control may be effected through a throttle valve inserted between the conduit and a vacuum source. Subsequently, on the conduit, a series of card rods may be arranged in the direction of rotation of the main cylinder.