The invention consists of eliminating the inner central shaft of the drum in order to allow a more homogenous mixing of the balls such that the chances of the balls being winning balls are not dependent on the position that they may occupy before the drum is rotated. To that end, the ball mixing blades (9) are integrally attached to the inner face of the structure of the drum. Not only does this structure allow mixing the balls better, but it also simplifies maintenance work on the device, in that it includes respective outer shafts (3) that are easily accessible without having to disassemble the drum in the event that the pin that links it with the mechanism for driving the rotation of the drum breaks.