The VT-172 is the American service Type designation for the commercial 1S5.
The VT-172 is a diode pentode valve designed for battery portable radio receivers. This short grid base pentode was designed with particular reference to AF voltage amplification. The 1.4 Volt filament is to be run from a dry cell and the 90 Volt anode rating matches standard HT batteries.
The bright nickel anode is oval in shape and hides the grids. The filament is a single strand that runs from the base connections, through the electrodes and out of the top mica to a spring support at the top of the valve.
The 1S5 is both printed onto the glass and etched into it. Possibly a re-branding?
The National Union name and Made in USA. The diode is formed above the top mica from a strip of metal and a support.
The thin glass tube envelope measures 18 mm in diameter and the excluding the B7G base pins, is 46 mm tall.
