Prototype set is here

[John Folsom]

Pete, Great photos and commentary. Almost as good as being there!

After seeing the photo of the number stamped on the rim of Nick's prototype CRT, I closely examined my two down-to-air metal bell prototype CRTs and discovered numbers on their rims I had never noticed before. One had D76, the other had D111. Both of my tubes have 14 pin stems and bases, which I believe places them somewhere earlier in the prototype tube progression. By March of 1953, RCA had provided 477 tricolor CRT samples to 177 companies.

I speculate that these rim numbers may represent a serialization of the metal bells used to assemble the tubes.

[Pete Deksnis]

Originally Posted by John Folsom

Pete, Great photos and commentary. Almost as good as being there!

After seeing the photo of the number stamped on the rim of Nick's prototype CRT, I closely examined my two down-to-air metal bell prototype CRTs and discovered numbers on their rims I had never noticed before. One had D76, the other had D111. Both of my tubes have 14 pin stems and bases, which I believe places them somewhere earlier in the prototype tube progression. By March of 1953, RCA had provided 477 tricolor CRT samples to 177 companies.

I speculate that these rim numbers may represent a serialization of the metal bells used to assemble the tubes.

I went back to my original photograph and checked for what I thought at the time were four integers. The first and fourth were lighter than the others and so I projected that they were hand stamped. Upon closer inspection, the first character could well be a 'D' with 205 following. If that is correct, and when he can find the time (Nick is young guy, not retired like us!), Nick might be able to confirm or deny my supposition. If so, this 20-pin prototype would fall right along the timeline.

Pete

[Sandy G]

This is all just so astounding...Like watching history being made...Yeah, I know, I'm silly...

[John Folsom]

Pete, I agree. When I get a chance, I will examine my other two metal bell prototypes. Unfortunately, they are both in sets (the Sparton and CBS prototypes), and getting to where I can read the rim numbers is a very non-trivial task. But I guess now I need to do it. One of these tube is a 6-bolt 20 pin base type, the other is a 3 bolt 20 pin base type. I would expect the rim numbers to be greater than Nick's if our theories are correct.

[Steve D.]

Phil Nelson posted on his great vintage TV site this CT-100 ad from RCA. Read the text for the amounts and brief history of RCA's prototype and CT-100 tubes made available to other manufacturers in the early '50's.This was not totally altruistic on RCA's or Sarnoff's part. They wanted color TV in the public's hands and if it took other manufacturers to get the ball rolling, so be it.

RCACT-10015GP22KinescopeAd.jpg
http://antiqueradio.org/art/RCACT-10...inescopeAd.jpg


-Steve D.

[miniman82]

I do believe the stamping said D205, but I could be mistaken. I'll have a closer look when I get home, and photograph it with my macro lens. I'm assuming all the talk of pin counts is important because as development progressed, the 20-pin 15GP22 was the ultimate result? So earlier on they were using different basing if I understand this right, which would place my tube later on in the development timeline and peg tubes with different pin counts as earlier ones?

[Pete Deksnis]

Originally Posted by miniman82

So earlier on they were using different basing if I understand this right, which would place my tube later on in the development timeline and peg tubes with different pin counts as earlier ones?

Something like that. I liken these exercises to riding an old mountain railroad for the first time: plenty of hidden curves ahead; somebody was there before us to build the thing, but it's all new now...

Pete

[John Folsom]

Nick, The earlier tubes had a 14 pin stem and base. I would speculate that this was because they had a 14 pin stem and base in the inventory. Later on, it became obvious that additional electrical isolation was needed between the convergence pin at [edit] 7-9KV and the other pins, so a 20 pin stem and base were developed, allowing 3 blank pin locations on either side of the convergence electorde.

I just checked my Sparton metal bell CRT, and it is a 20 pin 6 bolt CRT just like yours, with a type number of C73547, just like yours. My bell rim number is D255, and I believe Pete said you rim number is D205 (?). This all seems to make sense. My Sparton set is a early 1953 NTSC set, as best I can tell.

[miniman82]

Originally Posted by John Folsom

Later on, it became obvious that additional electrical isolation was needed between the convergence pin at 3-4KV and the other pins, so a 20 pin stem and base were developed, allowing 3 blank pin locations on either side of the convergence electorde.

Makes perfect sense.

Hey, speaking of convergence...

This thing has a long string of megohm resistors coming off the HV rectifier, with the convergence pot right in the middle and more resistors after it. It looks electrically just like the way it was done on the CT-100, except by that time they actually had a 50meg resistor to use in the circuit. What do your proto sets have for convergence?

[miniman82]

Update: took the tube out of the box, and it's definitely D225. Only 30 made between ours, if the assumtion of a production serial code is correct.

[John Folsom]

Thanks, Nick. Well it is all very interesting.

[Steve McVoy]

Your set doesn't appear to have a convergence transformer like the CT-100, so the question becomes how did they accomplish dynamic convergence, or maybe the question is "did they use dynamic convergence". I would be curious to know if there are other connections (capacitors to be specific) to the convergence grid of the CRT?

[John Folsom]

Nick, Here is a photo of the bottom side of he CBS prototype. It appears to use large high-voltage resistors, but it has been many years since I was under the chassis, so the details are vague. I have never had the HV cage cover off the Sparton, but as it appears to be a close cousin to the production Sparton set shown in Sames, I expect it will also have 50 megohm (or similar) high voltage resistors. Both of these sets are, shall we say, more mature than your set. :-)

[miniman82]

Originally Posted by Steve McVoy

Your set doesn't appear to have a convergence transformer like the CT-100, so the question becomes how did they accomplish dynamic convergence, or maybe the question is "did they use dynamic convergence". I would be curious to know if there are other connections (capacitors to be specific) to the convergence grid of the CRT?

That's exactly what I'm wondering. The other end of the resistor string has a wire that goes under the chassis, with connections to many different parts. So perhaps it does have dynamic, but it's done in a different way?


Thanks for the picture, John. I can see the 50meg resistors on that set, so you're right- it is more 'advanced' than mine.

[John Folsom]

Nick, Attached is a scan of the schematic from the June 1951 RCA symposium on tricolor picture tube development. While this features a RF HV supply and differs considerably from your sets configuration, it does show both vertical and horizontal dynamic convergence transformers. It would seem quite odd if your set did not incorporate some type of dynamic convergence.