In my experience 5751 tend to have a cleaner, more precise sound with "pinpoint" imaging. 12AX7 render larger images and can sound a bit more dynamic with punchier bass. Both nice; pick your poison or swap & alternate as desired. Mazda 12AX7 (silver plates) seem to nicely combine attributes of both, and strangely they have 5751 style plates. Avoid the Mazda/Cifte 5751 though - it's a bright mess.
Vas majority of circuits won't care a white about the gain difference, and in practice you can hear this as ~ 2dB less with 5751 (varies a bit by circuit). Just move your volume control up as needed. GE TM BP's are nice but my particular favorite was the "silver clips" variant produced just a couple years around ~ 1953. The "clips" were an over the top build that was deemed unnecessary very quickly. But these just seem to sound sweeter than other GE's which otherwise appear the same.
In my experience 5751 tend to have a cleaner, more precise sound with "pinpoint" imaging. 12AX7 render larger images and can sound a bit more dynamic with punchier bass. Both nice; pick your poison or swap & alternate as desired.
Vas majority of circuits won't care a white about the gain difference, and in practice you can hear this as 1 - 2dB less with 5751 (varies a bit by circuit). Just move your volume control up as needed.
I don't have a fraction of the "lore" and tube history some do. But I would say the "heyday" window is going to vary by tube type. Most tubes types won't last 10 years from introduction date, without being impacted by production cost cutting.
- 12AX7 - introduced 1949. Heyday mid - late 50s?
- 6SN7 - early 1940s. Heyday 1940s and early 50s
- Power tubes - higher power tubes came in later, especially 6550 and KT88. Some amazing power tubes were made through the 60s, and they were still quite good through the 70s.
- DHTs (e.g. 300B) - these are earlier than all above
Slyvania's quality unfortunately seemed to drop off the cliff sometime late 50s / early 60s. These tubes go from sounding amazingly dynamic, neutral, and 3D - to sounding bright, a bit lean almost solid-state like (typically gray plates). Culminating in the Sylvania / Philips ECGs of the 1980s. Even still, these late military tubes can sound good in some roles (particularly drivers / cathode outputs) - but they're a world apart from the "classic" Sylvanias of 50s and earlier.
