I'm going through my RCA CT-100 that came to me with someone's half-ass restoration...in going through and testing all the tubes I found that the 6BY6 Q synchronous detector had been replaced with a 6GY6/6GX6 at some point...while the set will demodulate lousy color showing that at the very least it is trying to work with this tube I don't know if this tube is an issue or if it is the lousy original resistors/peaking coils/alignment that is tripping me up and before I order a new tube or spend an hour digging through my unboxed used tubes I'd like to know if I need to...

Just looking at the spec sheets, they have different characteristics, and just happen to have pinouts that might just work, sort of.
https://frank.pocnet.net/sheets/049/6/6GY6.pdf
https://www.tubesandmore.com/sites/default/files/associated_files/6by6.pdf

Edit: at least a quick check of plate voltage would be worthwhile.

One of my tube substitution books (the one I could find) lists the 6BE6 and 6CS6 as subs for the 6BY6 so I've stuck a 6BE6 in for temporary. The 2nd video IF tube was wrong too, but atleast it was a type I had in the sorted portion of my tube stock.