The 3B7 is also called the 1291. It is a high frequency double triode rated for grid current in a Class C push-pull RF amplifier. The directly heated filament can be run in series at 2.8 Volts as well as the parallel connection at 1.4 Volts.
A battery portable transmitter operating at 25 MHz could generate 2.8 Watts. A VHF transmitter on 125 MHz would get 1.4 Watts out of the 3B7.
The lettering gives the Tungsram name but also that this valve was made in the USA and thus re-branded by Tungsram.
The anodes are bright but have extra heat sinking capabilities as the valve can dissipate 2.7 Watts per triode.
Side view. The central line is a rod and not a screen.
A clearer view of the twin triodes with central rod. This rod is probably connected to the centre tap of the filament.
The wide glass tube envelope is 29 mm in diameter and, excluding the B8B base pins, is 55 mm tall.
Reference: Datasheet. Type 3B7 was first introduced in 1949.