Sensibly equivalent¶ to:
The WD709 is a variable μ amplifier pentode and a pair of signal diodes in the one envelope. Probably this was designed as was the later EBF89 as final IF amplifier and AM detector. The other diode would provide AGC rectification.
Here is another example of this valve from Marconi. In this exhibit the glass foot and envelope are fused to leave a groove. The UK manufactured valves have a rounded base after the two types of glass have been fused together. I believe this groove construction indicates a Philips factory.
The thin glass tube envelope is 20 mm in diameter and, excluding the B9A base pins, is 56 mm tall.
References: Data-sheet & 1040. Type WD709 was first introduced in 1950. See also 1950 adverts.
Absolute Maximum Operating Conditions¶
Updated September 12, 2013.