So i'm writing a blues instrumental, and it's going well, but I have a problem. I can't think of any main "theme" that repeats throughout the song, which is a common trait in instrumentals. Is it really necessary though? SRV's version of little wing doesn't seem to have any "theme" and that's an instrumental..
Even if a song is instrumental, a theme should still be present. Of course, it isn't conveyed by the lyrics, but instead by the song itself. This can be done in a number of ways:
1) Chord Progression- Depending on the chord progression used, the song has a different mood to it. This is based on if it is major or minor in tonality, and the intervals between the chords.
2) Key- It doesn't necessarily matter what the pitch of the notes is, but as I said before, the tonality. Major often seems happy, while minor seems depressing, so keep that in mind.
3) Scale/scales Used- Based on what scales you formed your song around, it can give it a different mood to it. A Blues scale conveys blues, but it if you want to make your theme more specific, you can use something like a Minor Blues scale, for that different mood.
4) Dynamics- Dynamics help define the song-- it sets the mood as well. Forte (loud) can be used to define the message's power, while piano (soft) could convey sincerity. Crescendos and etc. are used to elevate to a climax.
5) Tempo- Something often overlooked, tempo can be utilized to create climax and a resolution. Faster tempo means more energy, and vice versa.
If it was EASY to play guitar , everyone would be like ME
And your comment of SRV-- something else a song can be based around is Style. SRV's unique style is constant throughout Little Wing. And the song itself does have a theme, but the style twists the theme. Behind the song, SRV's style communicates hardship and brokenness behind the "journey" the song takes you on. For example, Radar Love was a song originally done by Golden Earring. White Lion covered the song, adding their own unique style. The singing was higher, the guitar was heavier, but the theme was still the same-- however, White Lion's style added power and an intense feel to the song. The song still had a conflict, a climax, and a resolution. Basically, a song is like a book. It has a plot, a theme, and a conflict. If these things are present, the song takes the listener on a journey, making the song have more meaning, making it memorable. Lyrics aren't the only thing that accomplishes this-- instruments do as well. Classical compositions used only instruments to convey messages, or themes, so if you are struggling, try listening to a classical song and find the three main components I mentioned earlier. I hoped this helped!
If it was EASY to play guitar , everyone would be like ME
I think Little Wing does have a theme of some sort

something like

and the rest that follows it in the original hendrix solo
(excuse me if its wrong as I'm doing this from memory)

the part of the solo where they're all 3 are playing roughly the same thing

and SRV makes references to it all over his version too (I think he comes back to that part twice, can't be bothered to go check atm)

or if nothing else the chord progression is pretty recognizeable as a "theme"
I'd say yes. It's a lot easier to have something you can always return too... Theme, then go create tension, finish the sentence, and come back to the theme. Personally, it's a good idea, makes it a lot easier.
"The end result - the music - is all that counts"
It's up to you, it's your song. If you don't feel like creating a theme or you just can't come up with one, then don't put a theme there. There's not a rule that says you necessarily have to have a repeating theme.
It's really up to you. I'd recommend a theme, but there are plenty of good instrumentals that don't have one. A theme would make it easier to remember, and also, a great theme makes a good instrumental into a great instrumental.
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