The first tube in the pre-amp stage is a Groove Tube, as well as two other 12AX7A's found in the pre-amp section of the SVT-4PRO and one 12AU7 in the SVT-3PRO. Tube replacement may become necessary after one to three years, depending on enviroment, usage and care. Generally speaking, a tube does not need to be replaced unless it becomes noisy or microphonic. The SVT3-PRO contains 4 - 12AX7's and 1 - 12AU7 tubes.