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I think that would be difficult to answer unless someone was involved back then.

I'd guess that seleniums were still viewed as reliable. You wouldn't have to deal with the 3 amps needed by the 5U4 heaters, seleniums take up less space and run cooler.

I think the space issue was why the Sylvania 1-128 used selenium rectifiers in addition to a 5U4 and 7X6.

John

Quote:

Originally Posted by ohohyodafarted (Post 3029833)

Just out of curiosity does anyone have any insite as to why RCA used selenium stacks in the CT100 thru CTC4 series instead of 5U4s? :headscrat

Starting with the CTC5 and going forward, RCA switched to using 5U4s.

Data sheet for 5U4 indicates it will produce 225ma of dc current. I don't know what the ratings of the selenium stacks are, but my measurements indicate that a pair of 5U5s in parallel would be enough to supply the 440ma I measured in my CTC4. And further, the B+ fuse in the set is 450ma.

Here is a link to the technical data for an RCA 5U4. I must confess I'm not much good at interpriting these graphs.

http://www.mif.pg.gda.pl/homepages/f...49/5/5U4GB.pdf

Do you think this was an issue of reliability/pushing 5U4s to their upper limit, or cost in manufacturing considerations?:scratch2:

OH yes! Good point John. I forgot about the need for the seperate 5v filiment winding on the transformer. Now it makes more sense. I think this was more likely a matter of ecconomics than anything else. The transformers on the CT100 thru CTC4 series were already extremely large without adding an additional 5V winding for the 5U4's.

I would also say power consumption/heat generation was the reason. By the time the CTC-5 was developed, the tube count and power consumption were reduced enough. I bet the selenium rectifiers were more expensive than 5U4s at the time.

Actually, they went back and forth till silllycon was proven reliable. CT-100, 21-CT-55 and CTC-4 used selenium. CTC-5 had 5U4's, early production CTC-7's had 5U4's and later switched to silicon, and everything from CTC-9 onwards used silicon as well. I suspect that the development of cheap silicon rectifiers was a driving force in the engineers decision making process.