Why did Philco use different CRTs in same model in 1956?

[Tubejunke]

A fellow member and myself are in the process of swapping some parts. I have a 56 Philco tabletop that employs a 21ZP4B CRT that is on a brightener. I will need a CRT sometime although the set has had a brightener for years. Anyway it turns out that the other donor set employs a 21YP4. I already know the difference between the two. One is magnetic focus and the other is electrostatic. The question is why in the world would they do that? I understand upgrades and improvements but I dont see that as being any big upgrade. Just makes it more difficult from the repair standpoint and swapping parts. It would have been great to go pick up a complete set with everything I need. This difference puts a bit of a damper on the project although there are still other parts there that I need. Someone has a NOS CRT for $100 posted but I am skiddish about sending out a C note and receiving a box of glass fragments. Oh well! I will say this in closing Philco sometimes gets a bad wrap for low quality. At least I read that about Predictas. My Philco has served some 50 years and is just now getting to need attention. The CRT, and like everything else I have needs vertical attention. Whats with vertical circuits anyway. Are they under more stress than everything else in a set or something?

[Eric H]

I have a 53 Philco and it uses a 21EP4, it's magnetic focus but how it differs from a 21ZP4 I don't know. The book says there is no sub for the EP4?

[Tubejunke]

The only differences at all that are listed in my Sylvania Electronic Products Technical Manual are as follows:

The 21EP4 has what is referred to as a "Cylindrical Faceplate" and the 21ZP4 has a "Spherical Faceplate"
This doesent mean one is a "roundie" fellas because they are both listed and pictured as "Rectangular Glass Type" tubes

Maybe someone out there knows what the faceplate thing is all about because that is the only difference except for a difference in minimum usefull screen dimension of around .25 inch.

Except for the faceplate the tubes look identical but are not listed as subs.

[domfjbrown]

Originally posted by Tubejunke
The 21EP4 has what is referred to as a "Cylindrical Faceplate" and the 21ZP4 has a "Spherical Faceplate"

Just a guess, but is cylindicral like Sony's Trinitron - bending in only the horizontal plane, while the spherical one is more like any moden non-FST/flatscreen tube?

[andy]

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