The HA2 is a Marconi valve by M-OV that duplicates the American 955. The acorn valves were designed to have low capacitances and therefore work to higher frequencies than the normal valves.

Radio amateurs used them for work at 144 MHz after WWII.

The 955 was introduced by RCA in 1934 and the UK made devices did not achieve the success of the American product.

The tip to tip length is 32 mm and the envelope is 13 mm in diameter.

References: Datasheet. Type HA2 was first introduced^¶ in 1936.

 
Absolute Maximum Operating Conditions^¶

Triode	Vh	Ah	Va	Vg	mAa	ra
	6.3	0.15	180	-9.0	8	12000

Thanks to Frank Philipse for supplying the above PDF datasheet

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