The 1629 is one of the series of tuning eye tubes once available for use in radio receivers and test equipment as a means of providing a visual indication of a controlling voltage. For example, it was often used to indicate accurate tuning in radios as an aid to the listener. Also, some Heathkit capacitor-checkers used the 1629 to indicate the general worth of the capacitor.
Electrically, the 1629 is equivalent to the older type 6E5, but with two important differences: the 1629 has an octal base and a 12.6-volt heater, as opposed to the 6E5's six-prong base and 6.3-volt heater. It also features a sharp-cutoff triode instead of the usual remote-cutoff triode found in most of the series. Otherwise, the 1629 could be employed in any of the usual circuits designed for tuning tubes. It found its widest application in industrial and test equipment, and was a preferred type in systems designed for the United States military.
Due to the large production for the military, the 1629 remains readily available today; with minor equipment circuit changes, it can be substituted for the less-obtainable older tuning tubes with little difference in performance.
GE Essential Characteristics Manual, 1956 edition
Rheinschild, Philip A., “Tuning Eye Tubes in 1930s Radio Sets”, Philbert’s Place. <http://home.pacbell.net/philbert/tuning_eye/tun_eye.htm> 1 January 2003