Abstract: A gas burner for externally heating a glass tube which is positioned to be rotated on a rotational axis characterized by at least two multi-jet gas port blocks having a block axis with an arrangement of jets therearound being arranged with the block axes being on a radii extending from the rotational axis. Preferably only two port blocks are utilized and they are each separately displaceable relative to one another along the rotational axis and can be simultaneously displaced together along the axis during a heating operation. The arrangement of the plurality of jets on each of the front surfaces of the multi-jet gas port blocks is such that the jets are directed to a focal line which is at a distance beyond the rotational axis and a definite distance in front of the front surface.