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 KB/A identifies this valve as being made to specification K1001 or K1006, the B denotes qualification by a UK authority and the A identifies the maker as Mullard, Blackburn, pre Jan 1951.
Moving up from the base, the screening box connected to the cathode shields the two rod anodes of the diodes. The triode sits between the two larger mica discs.
A closer view.
The gap in the anode reveals the grid winding and the copper support rods.
The triode anode has a flange to one side that clamps round the support. The control grid supports have small square heat-sinks at the top.
The hard glass foot with the internal rods of the three part base pins.
The wide glass tube envelope is 27 mm in diameter, and excluding the B8B base pins is 61 mm tall.
