is a 6GY6/6GX6 a good substitute for a 6BY6?

[Electronic M]

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...