I succumbed. Idly clicking around the net. I just ordered a NOS 5692 RCA Red Base from 1971. Blame it on the fact that the RCA tube factory it was probably made in is 4 miles away. I’m attracted to local memorabilia. And the EBAY listing did show it testing good.
— Later Edit from 9/5 —
To continue replying to myself, my first impressions of the 5692 RCA Red Base tube is that it is a very slight improvement over the Tungsol 6SN7 “upgrade” tube that came with the Lyr3. I’ve only had about a dozen hours of burn in on the RCA, so it may get better.
Fighting the placebo effect, I think it is slightly quieter when turned up with no signal. It seems completely immune to effects of movement and has not displayed any microphonics. This is one of the design parameters of the 5692 as a “premium” over the standard tube.
Aesthetically, the Tungsol is better. It stands a bit taller, and you see much more tube glow. (see the RCA and the Tungsol together in my recent “What’s in the Box?”. The shielding in the RCA is heavier and obscures the view of the heater elements. However, I didn’t get it to look at.
The sound is very similar. I didn’t expect any night and day differences. I prefer having a locally made tube from when I was a junior in high school to the latter-day post Soviet product just because.