I've picked up two of these in as many weeks. The first one for $10 and now one today in trade for a rather used, but still usable blue Arcturus 180 tube. Actually, the one I just got still works and was working on a nearly dead 12BE6 tube! The model number is rubbed off, but someone penciled in T1240A on it and there is enough differences between this one and the T1243B that I picked up last week to make it worth keeping both. Though this MIGHT be a T1243A as it has a 35C5 like the T1243B unlike the 50C5 listed for the T1240A. Also, Antique Radio Museum is off on the transistor count on these. They list only 3 whereas even the faded tag on this one and my other example list something like five transistors on the tube diagram. It's not a Zenith, but a odd-ball radio that performs well enough. Sometimes I get a bit of a speaker or cabinet rattle, but that doesn't surprise me as these are actually a plastic table radio shoved into a wooden outer shell!
So, I can actually reduce the rattle by moving the inner radio around. Also, the speakers are held in by molded plastic clips cast into the inner radio shell. Yes, the speakers can work loose!
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I've never dealt with one of those radios,.
Usually when two similar radios use a 35v audio output vs 50v, one of three conditions are present.
1) Tube lineup has been altered.
2) There is/was a resistor or maybe thermistor added/deleted in the heater string that addresses turn on surge.
3)Someone subbed one for other.
Often citing higher line voltage, it's recommended to use a 50v for the 35v. Not one of my favorite mods.
I've tried the 50C5 sub on two radios, supposedly just a plug and play sub. Neither case worked and I had to go right back to to 35C5. I'll try and get a picture of the T1243B soon. Also, I'm going to see about getting that one worked on soon as well. Want to see how bad one of these might be to work on as they are usually dead on the FM side. The T1240A is the first one I've ever had that works fully on both AM and FM. I had one years ago that needed work, but turned my nose up at it and sold it with a batch of table radios I decided I wasn't interested in dealing with.
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.
Both radios I have use the 35C5, I can tell that much on the faded label on the currently functional unit. Sadly, the model number is not only faded, but someone inked in: T1240A and the Sams manual number over that area, so I can't make out anything. I know the one unit is definitely a T1243B as the label on it is great. I am suspecting the working radio might be a T1243A as it definitely shows signs of being a bit earlier in design. The T1243B has a dial light and there is a change in the controls as well. It has a Hi-Low tone switch where the Off-On for the FM-AFC is on working radio and the band switch is marked: AM FM-AFC instead of just AM-FM.
You might be on to something about the 50C5 swap. I am suspecting that though it tests well, there might be something wrong with the 50C5 I tried. I tried that same tube in both radios in place of the 35C5 and it just made them hum badly and play poorly. Note: These were two recapped radios that were playing fine beforehand and played fine when I put the 35C5's back in.
Apparently Sams did not cover your model(s). There is a T1245A that has four transistors, 35C5 & dial lamp but no provisions to adj tone. Because it has dial lamp, heater string was altered to include 35W4 tap in string, thus increasing total voltage. That allowed use of 35C5.