Calling All Tube Gurus

"If you can determine that a tube was made in the same plant, has the same construction and date codes, how critical is it if the tube was made by Siemens and rebranded as an Amperex? "

only critical to appearance if appearance is critical

"I’ve heard people say that if a manufactures tubes are not up to their standards, THOSE are the tubes they send out to the other manufactures for their branding. Fact or fiction?"

nonsense AND fiction. 

tubes are identified by and perform/sound similarly according to manufacture. rebrand is only as important as looks. even within one production run they vary in quality.

"matched," unless qualified by "gain matched" or mention of emmisions testing, refers to tubes of the same manufacture and contsruction testing within 5-7% plate current at spec voltage (power tubes) or 7-10% transconductance (preamp tubes) or both.

as far as branding/label, if you want them to look the same then hey it looks good. what someone writes on the outside doesnt alter origin, identity, sound or performance, nor does it opertain to matching.

Even on the same tube type, factories tweaked their construction from year to year. Usually to save money. Some of these changes are visible, others may not be?

Anyways, we (I mean the collective "we" here, including me) probably let our audiophile nervosa run rampant too much. I've used pairs a tubes a few years apart with different internal construction.

Example 1: GE 3-mica black plate 5751 from the 1950s, but one from 1953 with the "silver clips" and the other not (from a few years later) - and honestly it sounded great (Rogue Apollo tube monos). The "silver clips" was the better tube but enough of its magic came through without an obvious asymmetry here. Of course I ended up getting more (many more) of the silver clips version for symmetry and peace of mind anymore. But if you've got a factory match, "similar" construction, and a good electrical match, it should be fine.

Example 2: Brent Jesse might even slip you a slightly construction mismatch if you're not careful. I got a pair of Mazda 12AX7 from him, one with shiny silver plates (the typical kind for this version) and one with matte silver plates and a different hole arrangement. They actually sound just as good as any other pair of shiny plates I've got, so I kept them. 

Example 3: I also like the fabled Tung Sol round plate black glass 6SN7, but it's hard to find tubes that are strong. The heaters on these also commonly die (even if they tested good). So I've ended up now with a pair in my amp where one has the oval-mica construction and the other has round mica. Sounds great, no problems!

Mac new branded tubes are JJ’s. Not the best but reliable and available. 

First let me say, I’m certainly not an expert. I’ve owned a CJ amp for the past 13 years. After talking to a number of very experienced CJ owners with a plethora of institutional knowledge on the CJ owners group, I switched to matched tubes several years ago. All four of the output tubes on my amp are matched even though they are individually biased. I also matched the signal tubes. If you are interested in purchasing matched tubes, it’s important to purchase tubes from a reputable supplier with all the required equipment to properly test and match tubes. Kevin at Upscale Audio is a reliable source in the USA. However, his prices are a little higher and most come with a guarantee. Rolling tubes can get expensive. I’ve found talking to people that own or have owned the same equipment for an extended period of time to be the most helpful and reliable. In the end, only you can determine if the added cost of matched tubes make a sonic difference in your system. Luckily in my case, CJ has an online owners group that is extremely helpful. Again, a number of non judgmental people with a lot of institutional knowledge and experience. I’ve never run across a rude or condescending person on the CJ forum. Hope this helps. 

Back in the day when tube everything was more common, when a tube went out we would search Dad's tube drawer for one that was the same size and number of pins and replace it.  The biggest concern was trying not to too badly burn your fingers finding which one blew.  Seemed that whatever you dropped in sounded fine. Really just happy to get it working again to keep listening to the music.