Are there any tubes, especially those used in radios, with lower-case nomenclature? Would a 6SL7 or any other tube even have a version with a lower-case letter appended?
What’s the book’s audience? Would anyone reading that particular antique-radio-repair book be such a tyro that confusion would result? Would the reader/repairer think every tube mentioned in the book must have a lower-case s? Hardly.
What would you do if usage demanded a possessive pluralized tube number? Eschew pluralizing apostrophes!
According to Strunk & White (which gets no respect around here, though I think they have good advice in this case), one MAY use an apostrophe to pluralize something that is not a word. For example “Success has three s’s.”
I tend to omit the apostrophe even when something is not a word, unless the result is ambiguous. When talking about computer code, for example, or product names with mixed case, the apostrophe makes it clear that the ‘s’ is NOT part of the product name or variable name or whatever.
I’d like to file a complaint. According to these here instructions, I’m supposed to insert the 6SL7s. But you only gave me five of them. And after I inserted three of them, the casing broke.
I have started using them as invariable. There are two 12AX7 and three 6SL7 in the circuit.
I started doing this when I saw lower case acronyms, such as mips. In fact that one is especially problematic since I have seen phrases like one mip, as though mips were a plural. And you certainly don’t want to say 1000 mips’s or any other horror like that.
There is no such thing as “correct” style. There are conventions of style that are advocated by certain style guides that may be forced on certain people, such as newspaper employees or lawyers or students. Each of those sets of style will be correct even though they will contradict one another in literally hundreds or thousands of ways.
If you are not required to use a particular style guide you can never be accused of using an “incorrect” style. At least not by anybody who has a clue about the subject.
Style guides exist to provide consistency, because consistency is known to increase quick understanding. Without the existance of an overarching style guide, the best practice is to strive for what makes the usage most easily, widely, and quickly understood.
Using 6SL7’s for plural in this context is just fine. It satisfies the needs I listed above. It is not wrong by anyone’s standards unless brujaja has been told to follow a particular style guide and is ignoring that. Rewriting the sentence to remove any plural is also a possible solution - but it is not necessary to do so and sentences with plurals made obvious are equally valid solutions.
It’s just one example. My S&W isn’t handy, but it applies to anything that’s “not a word.” The definition of “word” is moot, of course, but arguably, an abbreviation isn’t a word, and a part number isn’t necessarily a word.
Anyway, according to Oxford, it’s incorrect, but according to S&W, it’s acceptable.
I stand by my rule: make it unambiguous. Other than that, when it’s not a word, you can use the apostrophe or not, as you see fit. Just try to be consistent. That makes more sense than following an arbitrary rule.