What gas is needed to solder silver? I doubt propane is hot enough. Will MAPP gas work? What flux is necessary?
When I worked as a radio and TV engineer before retiring and becoming a pro-brewer (yeah, I did that too), we used silver solder to bond copper ground systems for AM radio stations together. You need at least MAPP gas to get enough heat to melt the silver solder. Oxygen and Acetylene are better because the get higher temps, however are a bit more dangerous to work with. Since SS is a pretty good conductor of heat, it will take a while to get it hot enough (because of the large surface area of the keg) to melt the solder so patience is a virtue here.
Caveat: make sure that you do not have a ball valve that contains plastic or teflon connected to the fitting you are silver soldering. The heat will melt the insides and destroy the valve and they might catch fire as well.