RCA 6BY6/JRC-5915 manufactured in 1963/64. Mordy, do these look like your RCA 6BY6?
After 20 hours plus of burn-in, the sonic resemblence of these to the GE 6EB6 is immediately apparent. Like the 6EB6, these are very quiet tubes, revealing "great detail against a black background".
I would guess that as these tubes were often used to modulate and/or demodulate two separate signal streams, they would have to have very low noise and be very clean and neutral by design. And as these heptodes were often used to demodulate an FM radio signal down to audio, it is not unreasonable to conclude that they were also designed to faithfully process audio frequencies.
These heptodes are a great find, I think.
Again, borrowing from A11's description of the 6EB6:
Airy, open, spacious representation, neutral.
Treble: Extended, refined, not bright. Well resolved.
Check. But I think the RCA is a just a tiny bit brighter...
Mids: Quite sweet, round, clean.
Check. For both, vocals, horns and other acoustic and percussive instruments sound very natural
Bass: The bass quality is very good, well articulated and punchy.
Check. Both of these tubes have very good bass, strong, deep and punchy.
Stage: Large and wide
Check. Both of these tubes have a large, wide and deep sound stage, very realistic to me.
After spending a couple hours A/B-ing these tubes, I am a bit embarrassed to say that my old ears could not reliably differentiate between these tubes. I am quite sure there are differences, but I believe they are very subtle....
Again, "great detail against a black background". What really stands out in my mind for both of these is the amount of low-level detail revealed in my favorite recordings. It is kind of mesmerizing....
And perhaps it is not so bad that I can't pick a clear winner in that I can recommend both of these tubes without reservation. :)
Edited by gibosi - 5/19/13 at 11:06am