12AU7 and 12AX7 long black plates vs short black plates. Any difference ?

5751 are military with lower gain, not exactly 12AX7. 

@inna 

The 5751 has about 30% less gain than a 12ax7.  But don’t think it in any way translates to less versus more musicality.  I know they were also produced for the commercial versus military market.

Based on my use, I agree with Joe of Joe’s Tube Lore as they sound better than 12ax7 when used in the line stage of my preamp.  I have on hand and have enjoyed a number of brands of NOS 12ax7s but prefer the 5751s.

You may have an eye (or ear) opening experience and at a minimum gain insight by trying a good set of Sylvania black plate triple mica or GE black plate triple mica 5751s in your integrated.  These have been my favorite brands of the 5751s I have tried.  

Edit:  Or better yet maybe someone who has experience using 5751s in a VAC Avatar SE can chime in with their experience.

jetter, this could be interesting, if I can find them matched. I did try 12AX7 GE military short black plates and they sounded quite good but were no match for the Mullards that I have there. But right now I am saving funds for another quad of Mullard XF2 power tubes, two matched pairs I would say, matched quad is impossible to find or very expensive.

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. 

I choose more dynamic, that's, 12AX7.

Yeah, let's stay with classics

Mazda 12AX7 silver plates ?..Hmm, got to compare them with Mullard long plates one day.