I'm a 37-year-old male and I'm in to Rock/Heavy Metal singing. I would like to be able to sing higher with more power (as I'm sure every singer does). I discovered my head voice about 2-3 months ago, which seems like a step in the right direction. However, it seems quite 'weak' and lacking in stamina, so I've been trying to do some exercises, such as those recommended in this video by Ken Tamplin.
When I try to do the higher head voice exercises, I can reach most of the notes ok (although it is a bit of a struggle for the higher ones); however, my voice seems to tire quite fast when I do that. After about 30-40 minutes of doing those scales and practicing a few songs (not all in head voice), my voice gets tired and I seem to have trouble reaching some of the notes cleanly. Afterwards and the day after, I feel like some of the muscles around my jaw and high neck area are a bit stiff and tense. I seem to also suffer from some mild headaches for a day or two after practicing my head voice like this.
So, is this normal? I think the 'stiffness' is muscular, and I'm thinking it's probably because I'm new to head voice and because I'm using muscles that I haven't really used before. But, could it be a sign of vocal fatigue or that I'm doing something wrong? I'm not entirely sure if the best way to proceed is to practice it more frequently, or to give myself more of a break (a bit of a dilemma).
I have to admit that I don't have a vocal coach and I probably should get one.