1S4

The 1S4 is a miniature power pentode designed for the output stage of lightweight portable equipment. Tung-Sol points out that it can operate at relatively low plate voltages, an advantage in battery-powered receivers. Its coated filament is directly heated and requires 1.4 V DC at 0.1 A; the base is the seven-pin miniature type. An RCA sheet dated 1942 and a Tung-Sol sheet from 1947 document the same use, also providing a small historical reference to the spread of this family of portable valves.

The typical class A conditions show how available power changes with the supply: with 45 V on the plate and screen grid, the sheet gives 65 mW output into 8,000 Ω; with 67.5 V on both, 180 mW into 5,000 Ω; and with 90 V on the plate and 67.5 V on the screen, 270 mW into 8,000 Ω. The corresponding control-grid voltages are −4.5 V, −7 V and −7 V, referred to the negative filament terminal. These are example operating points, not outputs obtainable in every circuit. The stated limits are 90 V for the plate, 67.5 V for the screen and 11 mA total cathode current at maximum signal, falling to 9 mA with no signal.

The pin connections include a practical detail: pins 2 and 6 are both connected to the plate, while 1 and 5 are the negative filament connections and 7 is the positive filament connection; pin 3 is the control grid and pin 4 the screen grid. RCA also warns that some specimens may have the exhaust tip at the base end, so it recommends leaving the socket's centre hole clear. The Tung-Sol sheet also includes curves for triode connection, but the numerical output figures above refer to class A pentode operation.