The CV347 is a double diode triode with the CV specification based on the Mullard EBC21.
Traditionally the double diode triode sat at the end of the IF amplifier in a superhet receiver. One diode demodulated the AM signal which was then amplified by the triode. The second diode was the AGC rectifier and produced a negative voltage proportional to the amplitude of the IF signal. This negative voltage was then applied to the control grid of the controlled vari-μ valves and reduced their gain as the negative voltage increased. The receiver was thus sensitive for weak signals and the sensitivity was progressively reduced for stronger signals thus preventing overload and distortion.
The triode at the top is set at right angles to the screen that houses the two rods that form the diode anodes.
The triode control grid is supplied with heat radiating fins.
The grid winding on the copper rods and the central cylindrical cathode.
The wide glass tube envelope is mm in diameter, and excluding the base pins is mm tall.