20DP4 and 20CP4 are they interchangable

[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? If you have either one for sale that has good emission, also let me know.

[bandersen]

I briefly compared the datasheet and they look pretty darn similar. Same pinout, deflection angle, single field ion trap etc. Similar voltages and dimensions.

http://frank.pocnet.net/sheets/127/2/20CP4.pdf

http://frank.pocnet.net/sheets/127/2/20DP4.pdf

[drussell]

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?

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:

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.

[dieseljeep]

I always understood that was the slight differences in numbering codes was the manufacturers practice to differentiate their product, from the other makes.
Didn't all those CRT designs, have to be registered with the RMA or RETMA, at the time?
BTW,I never ran across, any of those CRT's listed in the text, like the 20" numbers.

[drussell]

Originally Posted by dieseljeep

I always understood that was the slight differences in numbering codes was the manufacturers practice to differentiate their product, from the other makes.
Didn't all those CRT designs, have to be registered with the RMA or RETMA, at the time?

They didn't HAVE to be registered, indeed many specialty tubes were only ever known by their manufacturer's internal number (like a Du Mont 5BP1 was known internally to Du Mont as a 2525A5). Many oscilliscope tubes, for example, never had a registered type designation (ie. the Tektronix T7100 1 GHz scope tube was never registered with the EIA), however, most commercially used tubes manufactured in any significant quantity were registered and received one of the standardized type identifiers.

I really can't say enough good things about this book... I have not checked but copies may still be available directly from Mr. Keller at <kellerp4(at)frontier(dot)com>. Quoting once again, this time section 1.10, "CRT and Phosphor Nomenclature", pp. 41-2:

Within the United States, receiving tubes, special purpose tubes, cathode-ray tubes and phosphors have been registered to promote standardization since the 1930s under the auspices of the Radio Manufacturers Association (RMA) which was founded in 1924. This trade association later became the Radio-Television Manufacturers Association (RTMA) in 1950, the Radio-Electronics-Television Manufacturers Association (RETMA) in 1953 and finally the Electronic Industries Association (EIA) in 1956.* The engineering council charged with administering the registration system went through its own share of alphabet soup starting at JETEC (Joint Electron Tube Engineering Council), JEDEC (Joint Electron Device Engineering Council) to the current TEPAC (Tube Engineering Panel Advisory Council).

The goal of tube registrations was to ensure common specifications and interchangability between manufacturers. In the early days of radio, one manufacturer's tubes might perform satisfactorily in a given circuit while another's might not, even though both had the same type number.

From about 1934 to 1940 the earliest cathode-ray tube registrations consisted of three and four digit numbers. The RCA 902 through 914 series, 1802 through 1813 series and the Du Mont 2501 through 2533 series were examples of the straight numbering system. Four-digit numbers were also used for some RCA CRTs, such as the type 4490 registered in the 1960s, although the reason for deviating from conventional practice is unclear. An "A" following the numbers indicated an improved version that could replace the original, e.g., type 902-A. From about 1940 until 1982, the tube type numbers were in the form of 5BP1 and 12AP4, where the first number designates the screen size in inches, five and 12 inches, respectively. The first letter is a sequentially assigned identifier to discriminate one tube from another of the same size, for example 5AP1, 5BP1 and 5CP1. This letter was assigned beginning with "A" and progressing through "Z" after which a second letter was added such as 5ABP1. The "P" followed by a one- or two-digit number indicated the phosphor screen type.** Eventually these reached P57 before being superseded by the new Worldwide Type Designation System (WTDS) in 1982.*** As with the earlier numerically designated tubes, a suffix with "A," "B," etc., designated an improved design which would replace the earlier version.

* Secrest, J.D. Electronic Industries Association: The first Fifty Years, EIA, 1974.
** EIA. "Optical Characteristics of Cathode-Ray Tube Screens," JEDEC Publication No. 16, June, 1960.
*** ------. "Optical Characteristics of Cathode-Ray Tubes," TEPAC Publication No. 116-B, November 1987.

He then goes on to discuss the specifics of the addition of the "V" in 1966 for viewable size after the FTC ruling and the change to WTDS in 1982 (screen size in centimeters, different phosphor coding, integral yoke option, etc.)

Originally Posted by dieseljeep

BTW,I never ran across, any of those CRT's listed in the text, like the 20" numbers.

That's not surprising. The 20s are quite rare as the 21s became the popular size and were manufactured in much greater numbers, hence the reason the SAMs for this TV lists the two 20-inch tubes (presumably they essentially used whatever they could source on the early models) and then the 21-inch version as they became more popular which would have required slight alterations to the faceplate and mounting but otherwise would likely be essentially the same chassis and cabinet.

[walterbeers]

Sounds like either one will work, so now I am looking for one that I can put into the Philco that I'm restoring. Set has been recapped, works ok otherwise, except the CRT went so dark, that one can barely see the picture. Emission is almost zero on the original tube. I got the Sams and ordered and paid $125 for a 21EP4, (which I have now posted for sale), as the Sams listed that as the replacement on the schematic, not realizing that some models used the 20 inch tube. Took out the old tube, found the NOS 21EP4 wouldn't fit. Now, I am in need of the 20DP4 or 20CP4. I thought I could retro-fit the 21 inch tube in it, but it won't fit the face plate, mounting corners would have to be cut out and redone, and then the straps that hold the yoke and magnet assembly are too short. Other words the 21 inch just won't work in it. Thanks everyone for you help. If anyone has one of these tubes that is usable, or would like to trade the your 20 inch tube for my 21 inch, let me know.

[drussell]

Originally Posted by walterbeers

Took out the old tube, found the NOS 21EP4 wouldn't fit. Now, I am in need of the 20DP4 or 20CP4. I thought I could retro-fit the 21 inch tube in it, but it won't fit the face plate, mounting corners would have to be cut out and redone, and then the straps that hold the yoke and magnet assembly are too short. Other words the 21 inch just won't work in it.

Ahh, that makes sense, as the 21EP4 is the first of the cylindrical faceplate tubes. One of the spherical faceplate 21-inch tubes might have been able to be wedged in there to fit OK but, of course, the cylindrical faceplate type one will be a totally different shape requiring different cabinet(s)...

[StellarTV]

I had a Magnavox once whose CRT was labelled as 20CP4/20DP4. It was a rebuild so I'd gather they were close enough to interchange.

[walterbeers]

Once again thanks for all the info, don't know if I will be able to find a reasonable and good 20CP4 or 20DP4 for it, but one never knows. I guess it's pretty rare.