Line-cord antenna coupler - safe on new cord?; tube substitutions

[Jeffhs]

I just reinstalled the line-cord antenna coupler from the old power cord onto a new one on my Zenith MJ-1035. The coupler fits the new cord and seems to work well, allowing the radio to receive all local stations in my area, but I am concerned about safety issues (that the cord could short to the coupler and cause a fire hazard, or destroy the FM front end by means of feeding line voltage directly to the grid of the RF amplifier tube [1/2 of a 17JK8] should the cord's insulation fail). I put a cardboard insulator in the clip per the original design. Did I miss anything, such as isolation networks between the radio's antenna terminals and the input of the FM tuner? The cord looks reasonably safe; in fact, I could see no cracks or bare spots on it.

BTW: The question I am about to ask has been in the back of my mind for some time. How is this line-cord coupler supposed to work? Obviously, it isn't connected directly to the AC power cord; moreover, there is a cardboard insulator between the cord and the coupler, so there is no actual contact between the coupler and the cord. The only thing I can come up with is that the coupler somehow connects the antenna terminal of the radio to the AC cord by means of capacity coupling.

BTW (2): I am thinking of eventually retubing my MJ-1035 from the ground up, even though the radio works rather well now (except for an intermittent volume control). I can probably find replacements for most of the set's eleven tubes, but at least one could be difficult to find (the 17JK8 FM RF amplifier/converter). Is there a direct substitute for the 17JK8 that might be more readily available? Another tube that might prove difficult to find is the 50EH5 audio output (the MJ1035 has two, one for each stereo channel). Would a 50C5 (which is still readily available) be a direct substitute for the 50EH5?

Thanks in advance.