as to maintain the tension of the delivered yarn substantially constant on a reference value t_ref the stock on drum 14 is controlled by a triad of sensors a first sensor s 1 preferably a hall sensor detects the passage of magnets m integral with flywheel 16 in order to determine the amount of yarn which is wound on the drum and the winding speed a second sensor s 2 preferably a mechanical sensor provides a binary information about the presence of a minimum amount of yarn at an intermediate area of drum 14 a third sensor s 3 preferably an optical sensor generates one pulse uwp per each loop unwound from the drum a speed evaluating block se processes signals uwp in order to calculate the yarn comsumption speed on the basis of the time interval between pulses uwp and generates an enabling signal le which enables tension control block tc when this speed is higher than a predetermined threshold value which is preferably equal to zero in the present embodiment an apparatus of this type is described in ep 2031106 to which reference should be made for a more detailed description a yarn recovering device 32 of the type described in ep 1741817 and shown in detail in fig2 3 is arranged between electronic brake 28 and tension sensor 30 yarn recovering device 32 comprises a reel 34 keyed to a drive shaft 36 of a motor 38 preferably a stepping motor or alternatively a brushless motor provided with an absolute sensor in order to measure the real position based on techniques conventional in the field reel 34 is arranged with its axis a slanting at a first angle a with respect to the direction of yarn f indicated by.