have heard of 5748WB or somthing similar as well,
Yes, there are several, like the 5702, CV4501 and CV4502, that are very similar though not the same.
BTW the military grade (JAN, WA, WB and such) is an interesting matter. Some say these are the same as the standard versions, only more rugged. Others don't agree.
Here's what Oliver Archut wrote about it in 2002 on the Neumann forum:
JAN tubes, Join Army Navy tube key, specialty tubes made for military use. Most of them were upgraded industrial tubes, with the one purpose to insure function under very heavy-duty circumstances. Their construction was designed to insure the minimal function over a long period of time. To put it into better terms, there were pre tested on the edge of their rating to sort out early failing ones. It doesn't mean that they were tested for audio, and most of those tubes are very noisy, not microphonic but a lot of random and white noise. In circuits like the U67 most of it is suppressed because of the HF cut off feedback as you pointed out. The higher noise relates to the cathode construction, some of them used sintered ones others a double layer coating design, the first layer is fast activated at the factory, the second one starts activating over a long period of time. *This process was patented by Sylvania and use for most JAN tubes after 1963* That is what generates the higher noise floor if used for audio. Sure they nearly live forever with hardly no microphonics, but gain structure is quite different, most of them sound quite harsh. In the last few years so many JAN tubes were decommissioned and entered the audio market, but for my personal taste a standard RADIO GRADE or some industrial tubes are more desirable. Of course there are some exception, like the JAN tubes than were special made for audio use.
For those who don't know, Oliver Archut is the same guy who makes the T49 tranny as used for this here M49 project. And yes, he does know a thing or two about tubes, too.
Of course, highly regarded mic builders like Dirk Brauner, the earlier mentioned Ben Sneesby and others do use military grade tubes with great success. Perhaps these tubes are the exception Oliver talks about in the last sentence of the quote. (?)
Here's a discussion that took place on another forum in 2005:
http://www.diyaudio.com/forums/tubes-valves/53836-difference-between-5687-5687wa-5687wb-tubes.htmlIt's just one discussion and simply the first that popped up.
For more in depth discussions about this (like Oliver Archut's story), one's best bet IMO would be to search in the archives of Klaus Heyne's excellent REP forum.
Come to think of it, our own forum may hold some useful ones, too.
Oh and as a rule the standard version tubes are cheaper.
January 10, 2021, 11:27:41 AM