@alexberger said: "NOS 5U4G RCA is not something special. I like Ken-Ran 5u4G. But Linlai 5u4g are on the same level and very cheap. I had a U52 GEC/Brimar brown base. It was really great, better than other stuff I had but it is crazy expensive now. I bought it when it was already used, and after 3 years it died. But to buy one today is too expensive."
The GEC u-52 is my preferred rectifier for my Allnic phono stage. I too had a Brimar branded brown base in box, fresh old stock as far as I could tell. Still has some life in it, but I retubed the entire system a few years ago and found an earlier iteration- also a "cup base" but black. I've read various things about these- that they were the industrial version, that they were earlier than the brown bases, that they don't have the finesse of the brown base. My experience has been that the black base is comparable. The tube has far more air in my set up than the Mullard metal base, which is another "grail" rectifier.