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The Ferranti ZD is a double diode. References ¹⁰⁴⁰ & ¹⁰⁴³ give the heater rating as 7 Volts at 200 mA but other sources place the heater requirement at 13 Volts at 200 mA. One has to be incorrect but when in a current limited series heater chain the effect is the same.

The bright box is a screen connected to the cathode. The diode anodes are the two rods seen either side of the cathode.

Thanks to Han Bex for sending these images.

The balloon envelope is  mm in diameter and, excluding the B5 base pins, is  mm tall.

References: Data-sheet & ¹⁰⁴³. Type ZD was first introduced in 1936. See also 1936 adverts.

Pin Connections

Absolute Maximum Operating Conditions^¶

Thanks to Frank Philipse for supplying the above PDF datasheet.