The CV1990 (commercial = 6SQ7) is a double diode triode sharing a common cathode, designed in the early 1940s specifically for superhet receiver use. The diodes would act as signal detector and AGC detectors, while the triode would be the first stage audio voltage amplifier.
As an American Type the the CV specification is the same as the commercial Type. This is typical of USA practice where a standard valves met the requirement.
The code GD is not in our list of makers.
The commercial Type designation.
The thin metal tube envelope is 25 mm in diameter and excluding the IO base pins the valve is 51 mm long.
References: Data-sheet, 1040 & 1043. Type CV1990 was first introduced in 1938. See also1938 adverts.
