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

Short plates and uncoated gray plates are indicators of the general trend towards cost cutting. In the span of a decade or 2, focus shifted hard from making the "best" quality tubes, to simply making them cheaper.

Some of these changes you can see via less materials and simpler construction: e.g. from welded to stapled plates in EL34; no more 5751 triple micas or retaining "clips" securing the plates to micas, no more massive "mirror" flashing on Sylvanias, loss of copper grid posts and heat sinking pieces. Some you probably can’t see (chemistry, metallurgy).

If you compare Mullard short plate 12AX7 to long plates from just a few years before, the long plates should sound a little more open, holographic, and articulate. The short plates just sound warm & thick, which is sometimes all you need from Mullard I guess. If you go a bit further back you’ll find long plates with the D / square getters which are even more revered.

Short plates can sound great but I think often if you can compare to the comparable long plates from just a few years earlier, you’ll prefer the latter. Bigger plates can mean more microphony too, though. The way these things are suspended in the glass envelope makes them a vibration antenna, and the mica spacers have to retain a tight grip over all these years & heat/cool cycles. 

mulveling, thank you. No short plates, no grey plates for me. 12AX7 long narrow plates Mullard that I have in line amp sound great, I have no intention to use anything else there. By the way, these Mullards don't sound thick or slow and they are not excessively warm.

Still, what about matte vs glossy long black plates ?

i just did a review about five different 12ax7a's that might be informative: Amperex Bugle Boy, Mullard short plate, GE short gray plate, Raytheon shiny black plate, Telefunken ribbed plate. i didn't include a Sylvania in the comparison because the only one i have is mostly used up

https://www.youtube.com/watch?v=JZRxlj48ctk&t=194s

@mulveling 

Interesting thread. I am currently auditioning a number of 12au7 preamp tubes and was wondering about the scope of the tube industry as it relates to what mfg did and did not do when demand was increased and then, later, when demand dried up. You know, adding features and then reversing course and going back to the basics. 

Is it accurate that the heyday of tubes was in the late 50's, early 60's? Having an assortment of tubes at my disposal I feel that much of the upgrades tube mfg undertook were in those years, you know, longer plates, getters, black plates vs grey, and other features that were once created to either make the product better or maybe just for marketing purposes only. Sometimes it is hard to tell, but there are subtle differences if you have a resolving system and the only tubes used are the preamp output tubes - and an SS amp.  

I am not looking for a history lesson (although if there is a book about it, I would read it!) here, but just a synopsis of how those features change the sound and what years you feel are the choice years for tubes.

The immediate experience I am having is that the Mullard tubes, despite some added features, such as long plates or red tips, tend to close inward with the added feature sets. Meaning that the midrange is accentuated, but at the expense of the high frequency air that the Amperex tubes provide. Amperex on the other hand, does kind of the opposite, IMHO. Amperex can be thin, Mullard can be thick, and there are many in-between s for each. 

Any input would be appreciated. I am trying others like RCA, Tung Sol, etc and am fascinated by the sound differences, but as a pragmatist I feel the need for a historical synopsis on the industry so that I can gauge my interest in a certain, hopefully, 10 year period or so of tubes in which to focus on.

I assume the best Sylvania tubes came from the early 60's, which may help the OP delineate what years to focus on to ensure best results, regardless if its a preamp tube or an amplifier tube.

@playbackmansion 

I can see how the Mullard won you over in the 12ax7 tubes. The higher gain provided in those tubes vs the 12au7, is probably a sweet spot.

In other words, when I listen to Mullards, the more volume I use the better the sound, which is the opposite of the Amperex. The Amperex are not bright, necessarily, but the top-end shine can be taken that way, especially at higher volumes. 

While it is an orange (preamp tubes) vs apple (amp tubes) comparison, the attributes of each are still readily apparent. Thanks for posting the video.