The U78 is a miniature HT full wave rectifier with two anodes around a common cathode assembly. As can be seen in the picture the U78 is identical to the 6X4.

At 325-0-325 input to the rectifier this will give an HT rail higher than is required for most domestic receivers. Use for audio amplifiers and small transmitters is most probable, or driven at 250 volts for radio receivers.

See also 12X4.

Osram is a domestic trade mark of M-OV and the paper label can be seen to the right on the envelope.

The thin glass tube envelope is 17 mm in diameter and, excluding the B7G base pins, is 57 mm tall.

References: Datasheet & ¹⁰⁴⁰. Type U78 was first introduced^¶ in 1959.

 
Pin Connections

B7G	1	2	3	4	5	6	7
	a(2)	-	h	h	-	a(1)	k

 
Absolute Maximum Operating Conditions^¶

Rectifier	Vh	Ah	Va	mAa
	6.3	0.6	325	70

Thanks to Frank Philipse for supplying the above PDF datasheet