Each end of an insulating tube which surrounds a transformer core projects beyond the electric circuits carried by the tube and is associated with an annular spacer member having two radially displaced bearing surfaces applied respectively against the core and the tube. An axial slit formed in the insulating tube is opened-out in order to insert the transformer core within the tube and then re-closed. A driving wheel is removably coupled to the tube in order to drive it in rotation during the winding operation.