This is old post, from Tube Asylum that could to some extend answer your question:
"This is mostly economics, IMHO, the more people who buy tubes
and they more money there is (and more competition there
is), the better the quality control gets and the more
effort is made to make better tubes. And indeed that's exactly
what's happening now.
40 years ago, a lot more tubes were being made for audio
purposes since nearly every audio amplifier from kiddie phonos
to stadium amps used them, and the large companies that made them
(like GE,WE,RCA, and Philips) could bring much more financial & engineering
resources to bear on the problems of tube design & QC than
today's smaller concerns can.
The other problem is that there's so many variables in
tube manufacturing that even if the same companies came
back and tried to make exactly the same tubes, they wouldn't
come out the exactly the same. Maybe close enough to
make most people happy, but you'd still get people saying
they could hear the diff.
I think the best you can hope for at this point is that
the current makers make the best tubes they can, in some cases
the results have been pretty impressive. "
Regards, Jack
"This is mostly economics, IMHO, the more people who buy tubes
and they more money there is (and more competition there
is), the better the quality control gets and the more
effort is made to make better tubes. And indeed that's exactly
what's happening now.
40 years ago, a lot more tubes were being made for audio
purposes since nearly every audio amplifier from kiddie phonos
to stadium amps used them, and the large companies that made them
(like GE,WE,RCA, and Philips) could bring much more financial & engineering
resources to bear on the problems of tube design & QC than
today's smaller concerns can.
The other problem is that there's so many variables in
tube manufacturing that even if the same companies came
back and tried to make exactly the same tubes, they wouldn't
come out the exactly the same. Maybe close enough to
make most people happy, but you'd still get people saying
they could hear the diff.
I think the best you can hope for at this point is that
the current makers make the best tubes they can, in some cases
the results have been pretty impressive. "
Regards, Jack