The 12F5GT is a high-μ triode for use as a signal amplifier. The heater is designed for series heater chains on 110 Volt mains. The 12F5GT would be a resistance/capacity coupled voltage amplifier feeding the output valve.
These valves were is short supply after WWII and Brimar offered suggestions for modification to use other valves.
Originally the Type was the 6F5 metal valve for parallel heater use at 6.3 Volts.
The The RCA logo and within the stitched anode.
The Radiotron brand, Made in USA and inside the anode is seen side on.
The wide glass tube envelope is 29 mm in diameter and, excluding the IO base pins, is 68 mm tall.
References: Data-sheet. Type 12F5GT was first introduced in 1939. See also1939 adverts.
