Okay I'm back!
The eternal struggle to “find the mix”. Believe me I feel your pain. As a young singer (completely fearless with no idea of technique) I too was able to sing up to Ab above middle C in a full chest voice. My teacher at the time told me this was very bad for my voice and proceeded to teach me his technique for singing “high notes”. Unfortunately this consisted of a lot of tension and I basically ended up sounding like a constricted cat singing anything above an Eb. This resulted in me developling a fear of high notes and a massive loss of range. What he was attempting to do was teach me mixed voice
Rule number one for finding mixed voice: Beware of tension in your neck. You must keep the muscles surrounding you larynx relaxed.
In regard to your falsetto. To test if it is falsetto or head voice start a slide at the top of your voice and slowly glide down into chest voice. Is there a massive “flip” as you hit chest voice? If so it is most likely falsetto. If there isn't a “flip” it may actually be head voice. Can you get louder and softer in the high part of your voice? It is very hard to alter the volume level of falsetto. Something to test.
The “hey” exercise that your choir leader got you to do is designed to get singers to make a noise outside of their normal speaking range. It is a handy way to help singers develop a belt technique however as you found out it won't help you realise mixed voice or head voice.
I'm going to give you an insight into what is actually happening at your vocal cords when you sing through chest, mixed and head voice. I apologise if I get too technical.
There are 3 parts to your voice that control the pitch of a note. Your vocal cords, your shortener muscles and lengthener muscles. When singing in “chest voice” your shortener muscles cause your vocal cords to contract and become “slack”. As you sing higher the lengther muscles should take over to stretch the vocal cords making them more “taught”. The problem you are having is your shortener muscles are stopping the lengthening process from occurring, causing you to carry your chest voice up until you cannot sing any higher.
I know you are sick of hearing “you should sing with mixed voice here” but as a general rule it needs to be introduced just above you natural speaking pitch. This will allow a much smoother transition into head voice.
There are certain conditions that need to be met before mixed voice can occur.
1. The larynx must remain low
2. Relaxation of the neck and dropping of the chin.
3. Increase in breath support
As a tenor you first bridge should occur around C#4. What I want you to do is hum up to this area (starting in chest voice) but no furthur. As you reach your bridge merge into a vowel of your choice. Only sing one or two notes higher, then return back into chest voice. When you sing the vowel try and add a “sob” or “cry” sound to your voice. This will help with the lengthening of your vocal cords and maintain a bright nasal sound.
The entry into mixed voice just above your speaking range is vital to your ability to sing into head voice. When humming never take it higher than your speaking range as this can cause tension.
I want you to spend a week focusing on this area. I suggest staying clear of your second bridge at F#4 for a while. What you will be doing is retraining your shortener and lengthener muscles to create balance in your voice.
I will continue this discussion and help you with head voice when you are ready.
I hope that can help you. Please keep me posted on your progress and if you are still having problems I will try my best to find a solution for you.
Ian 