Thank you Stevecham. That's great info.
I'll second viridian's recommendation of Viva Tubes. Good prices and great folks to deal with. https://www.vivatubes.com/
My other favorite source is Jim McShane. He doesn't have a pretty web site but just send an email or give him a call. He knows more about tubes than just about anyone, does meticulous testing and is a real gentleman. http://www.mcshanedesign.net/tubes.htm