With new tubes from China, Russia 50 hours is pretty close to runin
and not anywhere close to NOS tubes which take 2 x to settle in any last much longer the tube standards were far better ,such as 98+ % vacuum
vs 92-94% today , the metal recipe quality is not there back in the 50-60s grids many times inspected every 100 pcs now many every 3000.
the new Gold lions they bough5 the name ,not even close to the originals.
Tungsol made the Best 6550- today called kt88, even GE much better
or Japanese made from the 70s. That’s why I only buy or have custom built
by RaduTarta SET or transformer ,choke coupled preamplifier ,for quality throughout no China or Russian Anything. Emission labs makes nice tubes
as well as Western Electric 300B orTaks from Japan.