A plastic tube sealing device includes a clamp which has a pair of jaws that can move relative to each other for inserting and crimping a plastic tube, the jaws containing high-frequency (HF) jaw electrodes, and includes an electrical HF power supply circuit including an HF generator and the HF jaw electrodes. The plastic tube sealing device further includes an impedance control device (9) for acting towards maintaining an impedance of the HF power supply circuit constant during a respective welding operation by correspondingly controlling the variable impedance HF resonant circuit. For this purpose, the HF generator includes a variable impedance HF resonant circuit with a capacitor unit and a coil unit, and includes the jaw electrodes, wherein the inductance of the coil unit and/or the ohmic resistance of the HF resonant circuit is/are variably adjustable and/or wherein the capacitance of the capacitor unit is variably adjustable and the capacitor unit has an electrically controllable capacitance diode or at least one movable capacitance-altering capacitor electrode arranged in the clamp.
