6A4LA

The designation 6A4LA appears in Sylvania and Tung-Sol sheets as 6A4/LA, a filament-type output pentode for power amplification. Direct heating is 6.3 V, 0.3 A, with DC or AC filament operation under the stated conditions. Sylvania chiefly describes DC service in automobile receivers. It is an audio output solution for battery radios; both sources explicitly use the common designation 6A4/LA, without having to infer equivalence from names alone.

Tung-Sol gives two class A1 points. At anode and screen 135 V, grid −9 V: plate current 14 mA, screen 2.5 mA, transconductance 1.9 mA/V, approximate internal resistance 52.6 kΩ, amplification factor about 100, load 9500 Ω, output 0.7 W. At anode/screen 180 V, grid −12 V: 22 mA, 3.9 mA, 2.2 mA/V, 45.5 kΩ, load 8000 Ω, output 1.4 W. Both outputs correspond to 9% total harmonic distortion. Anode and screen 180 V are marked maximum.

Sylvania has its own characterization: at 135 V, plate 13 mA, screen 2.8 mA, transconductance 2.1 mA/V, output 0.7 W; at 180 V, 22 mA, 3.9 mA, 2.5 mA/V, 1.5 W, again at 9% distortion. Approximate amplification factor 150, internal resistance at the second point 60 kΩ. Do not combine a parameter from one sheet with the rest of the other point. Both sheets refer DC bias to the negative filament end; for AC, add 4 V algebraically to tabulated values and refer them to the midpoint.

Sylvania permits single-ended or push-pull use and recommends transformer or impedance input coupling. With resistance coupling, grid resistance must not exceed 0.5 MΩ. It stresses that a directly heated filament requires constant voltage for normal operation, distinguishing automobile receivers with 250 V plate supply, for which it proposes indirectly heated types 41 or 42. This application advice does not raise the 6A4/LA’s 180 V limit.

ST-14 bulb, medium five-contact base, marked 5B in drawings. Tung-Sol bottom view: 1 and 5 filament, 2 anode, 3 control grid, 4 screen. Sylvania specifies vertical mounting. Tung-Sol separately gives maximum overall length 4 11/16 inches (119.0625 mm), height above pin plane 4 1/16 inches (103.1875 mm), bulb diameter 1 13/16 inches (46.0375 mm), base diameter 1 3/8 inches (34.925 mm), all maximum. Documentation: Sylvania, Type 6A4/LA, Amplificateur de puissance pentode, p. 47; Tung-Sol, 6A4/LA Pentode Power Amplifier, plate 894-1, copyright 1940.