[35Z5]

Assuming all the other tubes have good emission, there is no reason a 50C5 will not operate in a series string set designed for 35C5. Usually if it doesn't, the 50C5 is bad or maybe one of the other tubes is borderline on emission. The 50C5 will reduce heater voltage on all tubes slightly, a borderline tube may not get hot enough to function properly.

There must be differences in models you've listed. The T1240A uses a 50C5, has total of six tubes and three transistors. It's basically a AA5 with hybrid FM section added.