Calling All Tube Gurus

I find it hard to believe that Siemens would intentionally make inferior tubes for Amperex, or vice versa. It’s an impossible concept. I have Valvos made by Amperex, and British Brimars made for German Lorenz with the German military hawk symbol on them. They work great.

Of course, we want our tubes to have matching codes, and names, but these days it’s becoming increasingly difficult to find. However, if they were made at the same factory within, say, a couple of years and have the same construction, I wouldn’t mind buying them even if they have different labels—as long as they’re matched, of course. In my experience, the sound will be indistinguishable from the originals.

@russ69 the only way to accurately match tubes is to match them in the circuit they are to be used in.

Here is a paragraph from Joe's Tube Lore. See if this is helpful information. For details, check the link.

Be careful of getting hung up on the brand name printed on the tube. In my experience most Siemens 6DJ8 family tubes you will find in the US are labeled anything but Siemens. Why? Siemens (and most other manufacturers) served as an OEM (original equipment manufacturer) supplying their tubes to many other tube manufacturers. While Siemens made these tubes they were labeled with the name of the company they were supplied to RCA or Phillips for example. Now not all RCAs and Phillips were supplied by Siemens, so that’s where points 1 5 on how to check tubes comes into play. Fortunately, in many cases the tubes are labeled with the county of origin & that can help you identify a tube’s manufacturer. Made in West Germany? It’s usually a Siemens. Made in Holland? Probably an Amperex. Made in the USA? Usually a Sylvania or an Amperex. Oh yeah a useful tip: RCA never made 6DJ8 family tubes. If it’s an RCA and has a ridged top it’s either an Amperex or a Siemens. I’d watch this brand closely if you stumble on it. Many tube dealers will sell lowly RCA 6DJ8 family tubes for $15 to $20 a tube without realizing that there is European made gold inside. Check the tube’s top for ridges, the label for where it was manufactured and if everything checks out, buy it! This is one of the absolute bargain brands in this tube type.

Amperexes were also frequently OEM’d to a variety of manufacturers. Most commonly to Mullard, DuMont, Beckman, Hewlett Packard and a few others. The risk with Amperexes is that they do vary by vintage and model and it’s hard enough to keep that straight among Amperex labeled tubes. Which vintage / version are you buying under another manufacturers brand? Who knows? It’s just pot luck.

https://www.audioasylum.com/scripts/d.pl?audio/faq/joes-tubes.html

Anybody know who makes McIntosh branded tubes?

To your date code question: I think it's not really relevant as tubes from the same batch can vary by a LOT.  I read that manufacturers think 5% variance is fine. Try 5% louder in your left channel and me that me 5% is fine. :)

I once replaced a pair of Sovtek 6550s that were damaged by red plating. I bought NOS Sovteks that were matched. Popped them in and viola! Louder on the left. Another tube- a Sylvania BH7A was causing the red plating. I replaced it with NOS. Soon after the right channel BH7A started acting up.  I realized that the best practices way to have avoided all this wasted time and money was to buy a whole tube set already matched from my manufacturer. "Your first loss is your least loss"