Doug Fattic 

Adding to what Gray said is that by tilting the angle of the flame up (you were probably pointing it down towards the tube) will focus the heat more on the boss than the tube. One of the challenges of brazing 2 unequal masses is bring them both up to brazing temperature at the same time. This happens by both the angle of the flame and the time heating each piece. As already mentioned too much of the tube was too hot and the silver followed the heat and collected away from the boss.

Another trick is that when the pieces are up to brazing temperature (be sure and know how to read the temperature phases of flux), place the silver to the left side of the boss and point your frame a few mms to the right of where the silver is placed. Then bring the flame over going left and just before it touches the silver it will melt off just the very small amount silver necessary. This way you don't melt off an unsightly blob. This makes it easier to pull the silver away just as the right amount melts.

jhodgson

Hey, thanks for typing that out Doug. I know you know your stuff and I appreciate the input.

I'm going to print this out and have my wife read it to me next time I'm practicing.

Nessism

I replaced a shifter boss for a guy a while back after it broke off. There was very little filler under the boss so be sure to take your time when installing braze-ons to assure you get good penetration. Some braze-ons have a little pocket on the contact surface, Llewellyn "Limpet" cable stops for example, so when I come across one of those I like to stick a little piece of silver in there before brazing. And speaking of braze-ons, I favor the stainless variety these days because they don't rust. Sweat (salt) just loves to collect inside cable stops and shifter bosses, as evidenced by the rust I've seen inside them before on some of my older frames. I'm not a particularly skilled brazer but even I don't have trouble brazing a stainless braze-on.