I dont think you can get some of those proprietary tubes anymore.
The current limiting tube comes to mind.
It was a great Radio. The best!
But it was a boat anchor of propriety Mechanical tuning and RF Slugs and Collins filters and tubes.
Even Rockwell Collins doesn’t have parts
http://www.r390a.com/Archived%20Pages/radio_rape.html
No, tubes are fairly the easy part! Now, anyway. They don’t wear out sitting on the shelf, and are a perfect vacuum. The internet changed things as far as anything obscure or collectible. You’re right about the current limiters though, they are pricey.
Art Collins grew up not too far from where I live. This area is very good for radio reception because the signals bounce up and down and wherever, along the mississippi valley.