the contact surfaces of the yarn guides then forms a zigzagged yarn path of travel along the groove it is possible to provide bar shaped spacers 140 which bridge over the bottom of an axial groove 112 i e the heating surface 117a 117b and define the yarn path at a precise distance from the groove bottom alternatively or additionally some or all yarn guides 132 may be provided with a peripheral guide edge for example a peripheral groove 142 fig3 a whose height from the bottom is adapted to the height of the yarn path that is predetermined by guide elements 140 in this manner the yarn which is guided in the groove is guided in addition by the side edges of the groove the peripheral grooves have the same depth over the circumference and thus are made concentric with yarn guides 132 however it is also possible to provide the peripheral groove with a depth changing over the course of the circumference for example in that the groove bottom is cut in circular cylindrical shape but off center relative to the yarn guides 132 this makes it possible to finely adjust the contact between yarn 1 and yarn guides 132 and define a zigzag yarn path by rotating the yarn guides to this end the yarn guides 132 may be rotated jointly and to a same extent for example by means of a linkage not shown to which they are connected the heater is preferably accommodated in an insulated box not shown in which it is embedded in a heat insulating material for example fiber glass the insulated box may be provided with a flap.