My hunch is that quality control was a bit higher in the 50s and 60s during the heyday of tubes. Once transistors came into the picture in a big way and really started to phase out tubes, then the quality of tubes dropped, perhaps due to cost cutting measures. This is speculation on my part.
You will almost always find that 60s tubes sell for more than 70s tubes, and in my experience, 60s tubes sound better, and have a more consistent sound from tube to tube.
You will almost always find that 60s tubes sell for more than 70s tubes, and in my experience, 60s tubes sound better, and have a more consistent sound from tube to tube.