New-Old-Stock (NOS) tube. The 50C5 is a miniature beam pentode primarily designed for use in the audio-frequency power-output stage of radio receivers. The tube features high poewr sensitivity and high efficiency at relatively low plate and screen voltages normally available in this application. The 25C5 differs from the 50C5 in heater ratings, and the 12C5 and 17C5 differ in heather and heater-cathode voltage ratings. In all other characteristic the four types are identical.