Why do old tubes sound better than current tubes?

Kirk, thanks very much for your informative and characteristically knowledgeable post.

To complete the picture, can anyone cite significant measurement experience that would provide a feel for the degree of section imbalance that can be expected for various makes of vintage and current production dual triodes?

Thanks,
-- Al

Almarg, a 5963 is equivilant to a 12au7. Would a pair of 12au7's Gm 1, 2800 Gm 2, 2850, NOS RCA Cleartop be in the ballpark of what you are referring to.

Orpheus, yes, what I am looking to establish is a feel for how widely Gm1 and Gm2 tend to differ from each other. And if any correlation can be established between the degree of that difference and whether the tube is current production vs. vintage, or among brands.

The 5963's I was referring to are RCA's date-coded 57-31 and 57-35, with long black ribbed plates and square getters. My tester indicates a nominal Gm for 12AU7's of 2200. The tubes in question typically measured about 2700 for one section and 2300 for the other, which means that one section was about 17% higher than the other.

In most cases the section having the grid on pin 2 was higher than the section having the grid on pin 7, but in one or two cases it was the opposite.

Regards,
-- Al

Thanks, Viridian. Anyone else?

Best regards,
-- Al