21ALP4A

The 21ALP4A is a direct-view black-and-white television picture tube. Tung-Sol’s May 1955 sheet covers 21ALP4, 21ALP4A and 21ALP4B: A and B have metal-backed screens, while only B permits higher anode voltage. Rectangular glass construction features a spherical grey-filter faceplate, external conductive coating and external single-field ion trap. Deflection is magnetic, approximately 85° horizontal, 68° vertical and 90° diagonal; focusing is low-voltage electrostatic. Sulfide-type P4 phosphor has white fluorescent and phosphorescent color with short persistence. Face-center light transmission is about 75%.

The heater requires 6.3 V at 0.6 A. For 21ALP4A the design-center DC limit shared by anode and grids 3 and 5 is 18000 V; the adjacent 20000 V applies only to 21ALP4B. Typical operation uses 14000 V, grid 2 at 300 V, grid 4 −55 to +300 V and grid 1 −28 to −72 V for visual extinction of a focused raster. Grid 4 DC limits are −500 to +1000 V; grid 2 is limited to 500 V. Rated ion-trap strength is 40 gauss. The source notes that brightness and definition decrease as anode voltage falls and generally recommends not going below 14000 V. The external conductive coating must be grounded.

Dimensions distinguish glass outline from usable raster. Overall length is 20 ±3/8 inches, 508 ±9.525 mm; bulb width 20 1/4 ±1/8 inches, 514.35 ±3.175 mm; height 16 3/8 ±1/8 inches, 415.925 ±3.175 mm. Bulb diagonal is 21 3/8 ±1/8 inches, 542.925 ±3.175 mm. Minimum usable screen is instead 20 1/4 inches diagonal, 19 1/8 wide and 15 high. These are not external dimensions or tube length. Values and tolerances come from Tung-Sol’s drawing, without scaling additional measurements from the image.

The small-shell duodecal base has six contacts present, B6-63, bottom-view 12L: 1 and 12 heater, 2 grid 1, 6 grid 4, 10 grid 2, 11 cathode. Recessed side anode contact J1-21 connects anode, grids 3 and 5 and collector. The socket must move freely or have flexible leads rather than rigid mounting; J1-21 alignment is referenced to position 6, ±30°. This is not an octal pinout, and six contacts do not mean only six base positions. Documentation: Tung-Sol 21ALP4, 21ALP4A, 21ALP4B, two pages, May 1955. The date belongs to the document, not an inferred first sale.