The 18S4 is in some ways a bit of a Dog being it requires separate tuned networks for each channel, a strange almost never available antenna tuner and has a broad IF Please excuse me for suggesting but think you would be happier running something like a Collins 618S1 being that gives you 100 watts of plate modulated AM, requires no special tuning networks other then crystals, uses the standard 180L antenna tuners that are all around and has a mechanical filter in the IF that helps in reception, at least on AM.

I have done all three, the ARC-38/RT-311, 618S1 and ARC-38A and unless you got to have a 18S4 I think that’s the way to go, but that’s just me.

Additionally I know someone who has been trying to sell a 618S1 and may be willing to deal.

Ray F/KA3EKH