Newb Tube question

With vintage small-signal tubes, long plate variants are usually considered more desirable. Long plates and short plates are not specific to a manufacturer, but they are generally used in reference to 12ax7 or 12au7 type tubes. "Long" is generally around 19 mm, and "short" about 16 - 17 mm. In most cases, a manufacturer produced long plates in their early-year runs of that type, and then later switched to exclusively short plates (late 1950s - 60s). In general, earlier vintages from any given manufacturer contain features that are considered more desirable (but it can be hard to decouple this from the fact that earlier vintages themselves are almost ALWAYS more desirable in vintage tubes!) - other examples of this include specific kind of getters (square, "D", or double halo getters versus later single halo getters), black plates vs. grey plates, triple micas vs. double micas, pinched waists vs. straight tubes, larger and prettier getter flash patches (e.g. Sylvania "chrome dome" 6sn7), etc.

The "smooth plates" are a pretty unique feature and pretty much always refer to the Telefunken 12ax7/12au7 tubes. Those are excellent tubes. Ei (Yugolsavia) later bought the old Telefunken tooling and made smooth plate tubes for some years - these are good tubes too, but perhaps not as good as the original Teles.

With new tubes you can find tall and short plate variants of 12ax7/12au7. If I were to generalize (though probably badly badly), the tall plates tend to give a sweeter sound but the short plates could be less microphonic (less to rattle around).

Not long ago I bought a quad each of Upscale Audio’s Platinum Grade for the following Russian re-issue tubes: Mullard, Tung-Sol gold pins, Gold Lion. For use in my VAC Renaissance phono stage. All of them sound really good! But yes I slightly prefer the long plates (Mullard, Gold Lion) to short (Tung-Sol). The Gold Lions are particularly nice. The Mullards are great for systems that need help taming an aggressive treble response.

I also tested these in my MaxPreamp2 tester, and yes both long plate types show notably higher transconductance than the short plate Tung-Sol (+10% - 15%). The Tung-Sols weren’t weak, though - in fact they’re still a good deal stronger than most of the vintage/NOS 12ax7 in my large collection.

That said, even though they measure a lot lower in transconductance, my favorite vintage 12ax7- the 1960’s Mazda chrome plates - still whip all 3 of the the new tubes in detail. I’m currently using a mix of Mazdas and the tall plate Russians, since too much Mazdas causes too much top-end energy.

12at7 and 12ax7 are not common subs, but some circuits do allow that in certain positions. The Herron VTPH-2A is one such example - using 4 of 12ax7 and a single 12at7 yields another 5 -6dB or so of gain over 2 of 12ax7 and 3 of 12at7. The 2 slots allowing the sub may be mu followers? 12ax7 are not usually used in follower positions, though I guess some phono stages do that. In that case a 12at7 should be a fine sub.

Definitely don't swap them unless you know it's an OK choice for that slot.