The AEG K1 like the Siemens Type A was developed as a telephone repeater triode. The K1 filament is in the form of a hairpin where the emission was at a maximum at the tip. The Type A has a broad filament supported at the sides and gave a more even distribution of emission. Both types date to 1917.
Below the valve can be seen a glass encased resistor.
The note giving the filament current. The filament would be fed from a 6 Volt accumulator with a series resistor to limit the current to the given value.
The original box dated to 1918.
Thanks to Joe Gruber for sending the images.
The balloon envelope is mm in diameter and, excluding the base pins, is mm tall.
Reference: 3002. Type K1 was first introduced in 1925. See also1925 adverts.
