Abstract: There is provided a tube amplifier comprising a first vacuum tube K3 having a plate applied with a positive voltage and a cathode applied with a negative voltage by a first resistor, a second vacuum tube K4 having a grid connected to the cathode of the first vacuum tube and a cathode or a heater applied with a voltage close to a potential of zero in relation to a plate voltage, and an output transformer TF1 having a first coil and a second coil which is connected to a loud-speaker, said first coil having a terminal connected to the plate of the second vacuum tube and another terminal applied with a positive voltage. Also, timer means is used to optimally maintain the grid voltage of the second vacuum tube when the power source is turned on, and the plate voltage of another vacuum tube K1 provided in a preceding stage of the first vacuum tube is controlled to maintain the negative voltage applied to the grid of the second vacuum tube at a desired value.