Unfortunately, I think it is best to have your power tubes matched within reason. I'm sure the amp manufacturer or distributor would be happy to tell you for sure. If you know what the 300B you have measures, maybe someone could sell you a match for it.
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I would always buy a matched pair. Considering the future, most likely one going at a time, preferably a matched quad if money is easy (hah).. In either case, having your one good used one will give you the luxury of time in the future.
I assume you like the sound of your existing brand/version. Exact brand/version available so no surprises and spare is the same?
Want to try different brand/version? Buy from a source that will allow returns or exchanges. Different brands/versions do sound different. I am now exchanging some NOS Sylvania’s I bought and tried for some NOS GE’s I know I love the sound of. Brent Jesse lets you break them in, and exchange them if you want, others will also.
Break them in before you do critical listening. You can just turn/leave amp on, nothing playing. After 3 days, that is how they will sound.
Different brand? After breaking them in, as a test, I would put one new one in with your used good one. Balanced? Sound Great together? Have a method to listen to each side individually? Learning about the sound of each brand is good info.
if each 300b is biased manually does it really make that much difference going matched when only one tube goes bad??? I have just swapped out the "bad" tube for many years and never heard the difference. the amp I have now is self-biasing, which I really like and no more hand injuries doing manual biasing for me!
the tubes I am sending back are 6sl7’s, from my integrated amp. Surprisingly they effected the sound substantially. the amp also has 6sn7’s, have not replaced them yet.
the original tubes say ’Cayin’, the manufacturer, so I don’t know their specific brand. Happily, the GE sounds as good as the Cayin labeled one.