I've seen a short video on this process several times on TV. It's part of a series we get called How It's Made. One thing you would need to look into is whether the tubing you have in mind would tolerate these temperatures with thin wall tubes without gas purge. In the video I saw they we using a flame, and flame concetrates the heat, and also can be run so that there is a gas shield effect. These were also pretty low quality frames so they may have been rugged enough. One get's some oxidation at 900 degrees. I notice that california brazing has some pressure vessles.