M-OV's first indirectly-heated HT rectifiers were the MU12 and MU14. These were in fact the same valve but only a minority of each production batch were good enough to withstand the full rating when tested. These few were labelled MU14. This exhibit came in a wooden box as spares for a Marconi crystal calibrator.

The classic envelope is 43 mm in diameter and excluding the B4 base pins, is 103 mm tall.

References: Datasheet & ¹⁰⁴⁰. Type MU14 was first introduced^¶ in 1933.

 
Pin Connections

B4	1	2	3	4
	a(1)	a(2)	f	f

 
Absolute Maximum Operating Conditions^¶

Rectifier	Vh	Ah	Va	mAa
	4.0	2.5	500	120

Thanks to Frank Philipse for supplying the above PDF datasheet

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