Patent ID: 7583140

Claim:
An amplifier circuit of a vacuum-tube amplifier, which receives an input signal through a grid of a vacuum tube for amplification, receives a first voltage through a plate of the vacuum tube via a first resistor, receives a second voltage through a cathode of the vacuum tube via a second resistor, amplifies the input signal and outputs the amplified input signal through an output node connected to the cathode, comprising: a resistor for dividing the first voltage with the first resistor and applying the divided voltage to the plate; a power amplifier having an output terminal (−) connected to the voltage-dividing resistor for controlling connection/short-circuiting of the voltage-dividing resistor in response to the voltage of an input terminal (+) of the power amplifier; and a differential amplifier having a non-inverted input terminal (+) receiving the voltage of the cathode of the vacuum tube through voltage-dividing resistors and a time constant condenser, and an inverted input terminal receiving the voltage of an output terminal thereof through voltage-dividing resistors and a time constant condenser for continuously controlling the voltage of the input terminal (+) of the power amplifier according to differential amplification to converge a DC voltage of the cathode on 0V.