the source yarn etc the plug height level normally may stray outside the desired control limits even though the speeds of roll 7 and roll 18 remain constant during operation and there is no yarn slippage therearound to control the plug height level device 21 is placed in the yarn path between roll 18 and the point of yarn withdrawal from tower 13 device 21 comprises base 22 a plurality of rods 23 extend upwardly from and are securely mounted in base 22 the lengths of rods 23 decrease stepwise in the direction of yarn movement as will be more apparent in fig5 7 rods 23 are inclined alternately in one direction and then the other direction to form a slightly crossed configuration for receiving the yarn therebetween the yarn passes between the rods turning one way and then the other thus when the level of the plug height drops the yarn will tend to assume the path indicated by bottom broken line 24 thereupon the amount of rod surface in contact with the yarn proportionately increases thus giving rise to an increase in tension on the yarn between roll 18 and device 21 but a decrease in tension in the yarn between device 21 and tower 13 with such decrease in tension the level of the plug in tower.