The 7X7 is a double diode triode but unlike most such valves the 7X7 has a separate cathode for diode two adding to the flexibility of use.

The diodes can be seen above the base with the amplifier triode at the top.

The side view of the diode anodes.

The heater passes up through the two cathodes in a single vertical rise.

The wide glass tube envelope is 29 mm in diameter, and excluding the B8B base pins is 70 mm tall.

References: Datasheet & ¹⁰⁴⁰. Type 7X7 was first introduced^¶ in 1948.

 
Pin Connections

B8B	1	2	3	4	5	6	7	8
	h	a(t)	g1(t)	k	a(d1)	a(d2)	k(d2)	h

 
Absolute Maximum Operating Conditions^¶

Triode	Vh	Ah	Va	Vg	mAa	ra	gm
	6.3	0.3	300	-1	1.9	67,000	1.5

Thanks to Frank Philipse for supplying the above PDF datasheet