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Originally Posted by walterbeers
I have a Philco console (cir. 1953) that uses either a 20CP4 or 20DP4. Number is missing on the old CRT, and Sams lists both, along with a 21 inch tube that I found out doesn't fit. Are they interchangeable?
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As Bob noted, it appears they should be interchangable... Same size, spherical faceplate, magnetic focus, electrically virtually identical, etc. There are different versions of both, though (ie. with/without external aquadag) so if you can't find the exact one for your application you might have to add a HV capacitor or whatever but that shouldn't be a major problem.
I expect the "differences" were likely just slightly different versions of essentially the same thing with minor variations being "invented" at different companies and thus being registered separately with the RTMA.
In Peter A. Keller's "The Cathode Ray Tube: Technology, History and Applications" (1991) pp.158-160, he notes:
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Twenty- and 21-inch rectangular picture tubes entered the scene in great profusion in the 1950 to 1951 period and the 21-inch screen rapidly became the dominant screen size for not only console models but some table model receivers as well. The first tube was the magnetically focused Hytron 20CP4 followed by the Sylvania 20DP4 and the electrostatically focused Hytron 20FP4, RCA 20GP4, Rauland 20HP4, Du Mont 20JP4, General Electric 20LP4 and RCA 20MP4. Several 21-inch glass tubes with conventional spherically shaped faceplates and 70-degree deflection angle also were produced including the National Video Corporation 21WP4 and 21XP4, Westinghouse 21YP4 and Sheldon 21ZP4.
Another screen style, the cylindrical faceplate, emerged in 17- and 21-inch picture tubes in 1951. The faceplate was shaped like a slice out of the side of a cylinder with the screen flat in the vertical plane and curved in the horizontal. This screen style was an attempt to eliminate the curvature in the vertical direction which tends to reflect ambient light toward the viewer regardless of the height of the light source above the floor. By tilting the cylindrical picture tube's face slightly forward, reflections were directed downwards, away from the viewer's line of sight. Cylindrical faceplate picture tubes began with the magnetically focused National Union 21EP4 and 17QP4 and their electrostatically focused counterparts, the 21FP4 and 17LP4. Others in the series included the 17SP4, 17UP4 and 21KP4 by Thomas Electronics, 17VP4 and 21JP4 by General Electric, and 17YP4 (Figure 5.37) by Philco-Lansdale. By the mid 1950s, the cylindrical faceplate had fallen by the wayside and it was not until Sony revived it for their highly successful Trinitron color picture tube (see Chapter 6) that it was used again commercially.
It should be noted that by the time the rectangular picture tube was introduced, gray glass faceplates had become universal for contrast enhancement, thus all of the rectangular glass picture tubes employed them from the start. Still, variations of the rectangular tubes were produced with contrast and brightness improvements including etched (or frosted) faceplates, aluminized screens and combinations of the two. Usually these variations were identified with suffixes from A through D. There was no consistency in suffixes between differing tube types, they were simply issued on a first-come basis and it cannot be safely inferred that a 20CP4-A possesses the same feature configuration as a 17KP4-A. Differences in external conductive coatings and ion-trap magnets were sometimes identified by the same suffixes.
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