Musical instrument technicians spend years learning and practicing their trade, but for simple repairs, and with the right materials and equipment, just about anybody can do some simple brass instrument soldering. There are basically two types of soldering involved with brass instruments: (1) joint soldering if an instrument has received rough handling through marching and travel, and (2) soldering of braces, finger rings and lyre holders that have come off. These are easy repairs that do not require a lot of knowledge.
Clean both parts that are to be soldered with a very fine emory cloth. Wipe off any dust and debris with a clean, soft cloth.
Use a small stiff-bristle wire brush to apply flux to both parts that are to be soldered. If two tubes are to be soldered together, join them. If one piece of brass is to be joined to another piece of brass, place the piece that needs to be soldered in place and secure with a metal clamp.
Use a propane torch to heat the pieces that are to be soldered until the solder flows into the space between the two pieces. (Brass melts at a very high temperature, so you aren’t going to melt the brass with the torch.) As soon as the solder starts to melt, remove it to avoid accumulating excess solder and having it run out of the joint.
Allow the area to cool completely and wash off any excess flux with cold water and dishwashing detergent.
Avoid putting the flame directly onto the solder; when the metal is hot enough, touch the solder to the area that has been coated with flux and it will flow into the space.
Work in a well-ventilated area and avoid using solder that contains lead.