[link to original on tumblr]
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I'd always thought that "Hold Your Head Up" was in D major, but if the bass part holds to that 5th, 7th, octave phrase, it's in D minor. (It's A, C, D.) So I'm unsure of the key, but it seems like the bass part does retain that tonality (just transposed down from "Time of the Season"'s E minor). The rhythm is different though.
I also figured out a bit of the guitar part because I thought it would be boring to have just the bass part. Way back in October 2011 (which is - I think - when I was first really getting into Argent), I'd thought I'd figured it out (I still have an erroneous tab that I wrote in a notebook for a literature class I was in at the time; it's right under my notes on the Bhagavad-Gita), but then I realized that I was just playing note pairs an octave apart and that the actual part was more involved. I think I have it now, although my guitar tone doesn't match the original very well.