The 13D1 is a double triode and is a special rugged variant of the commercial 25SN7GT.
The data-sheet for the 6SN7GT indicates the usage of this medium μ double triode is as an RC coupled amplifier or a TV vertical oscillator and output valve. In the latter case one anode can be run at 3.5 Watts dissipation providing the total dissipation remains at five Watts maximum.
The other side carries the CV423 Type designation and KB/?? for the maker. The remains of the last letters are two vertical bars.
The valve uses single mica discs but also caps on the main supports. Most rugged valves used double mica discs as the extra strength resisted the shaking supports from enlarging the holes in the micas.
A better view of the construction. The heater wire passes several times through the cathode.
The wide glass tube envelope is 29 mm in diameter, and excluding the IO base pins is 656 mm tall.
Reference: Data-sheet. Type 13D1 was first introduced in 1937. See also1937 adverts.
