I'm a retired electronic (solid state) engineer trying to design stuff from scratch using tubes. As an initial techie question: After perusing RCA tube books, I can't find an individual pentode with properties similar to to the one in a 6U8; but there many 6U8 clones. The reason I pose this for a tube designer is that new tubes were very difficult to design and a number of modern tubes were just old designs in a new envelope.

Since receiving tubes have not been manufactured, much less designed, in the U.S. for many decades, the designer you seek is likely outside the U.S., or long dead.

Yep, I'm a retired EE and in the early 70's when I was in college, tube design was by then long discontinued.

Not to say that there aren't a few hardy survivors of the golden age of tube design; but they would almost have to be at least in their mid-80's.

I'll suggest that if there are tube engineers still living, they are originally from some part of the former Soviet Union. The Soviets used tubes in their military hardware for a good 20 years past when vacuum tube use was abandoned in the West; so to the extent that "support engineering" was still required for the manufacture of tubes long after the need for same ceased in the USA, there still may be a few elderly tube experts around somewhere in the world. Many Soviet tubes intended for consumer electronics were functional copies of existing USA/Euro tubes but they still needed practical experts to manufacture them.