The X78 is a triode hexode mixer. The triode has to supply 10 volts pk-pk to drive the hexode mixer correctly.

The paper label shows evidence of a hard life.

The thin glass tube envelope is 17 mm in diameter, and excluding the B7G base pins is 45 mm tall.

References: ³⁰⁰² & ¹⁰⁴⁰. Type X78 was first introduced^¶ in 1949.

 
Pin Connections

B7G	1	2	3	4	5	6	7
	g2,g4	g1	h,k	h	a	a(t)	g1(t),g3

 
Absolute Maximum Operating Conditions^¶

Hexode	Vh	Ah	Va	Vs	Vg	mAa	mAs	ra	gm
	6.3	0.3	250	75	0	4.5	3.4	0.7M	0.78

Triode	Va	Vg	mAa	gm
	100	-1	10.0	2.8