The 247 amplifier uses a NOS (New Old Stock) tube in a cascode amplifiers design, the worlds only microphone with this type of circuit. Cascode amplifiers were originally designed for the TV and radio industry. When used as an audio amplifier, cascode circuits allow a low noise, linear response without the use of any type of feedback within the circuit. When pushed into distortion, cascode amps have some of the sweetest tube saturation of any amplifier design, including class A types.