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.