The 7F7 is a Loctal (B8B) implementation of the Octal based 6SL7GT double hi-μ triode. The 7F7 has a 6.3 Volt heater and the 14F7 is designed with a 12.6 Volt heater.
The gettering covers the inside of the glass but the twin triode section can be seen. The anodes are stitched together. The locating pip guides the equi-spaced pins into the base and the notch on the spigot pushes into a stiff spring that locks the valve in place.
The working area of the triode is small, the oversized anode helps to dissipate heat.
The identification is on the dome.
The wide glass tube envelope is 28 mm in diameter and, excluding the B8B base pins, is 54 mm tall.
Reference: Data-sheet. Type 7F7 was first introduced in 1940.