That black coating is a special paint made by only one company on the planet and it’s a heat sinking paint. It’s very expensive paint / coating.
Some tubes have it and they are all expensive as one of my favorite driver stage tube which is EF12, first seen in Klangfilm F2a amps.
Really. I wasn't aware about this. Sounds interesting.
 
	 
  
 
		 
		 
			 
 
		 
 
		 
 
		
 
 
		 
 
		