Originally Posted by celika

so i've heard about the butane/propane instead of acetylene, with oxygen - or even better a oxygen concentrator. No high pressure gas, some investment at the begining but cheap to run and easy to refill,... I've made search about it, and it seems to work quite well to braze bikes.

There is some questions that my search didn't answer surely:
-it seems you can use a "classic" acetylene torch, in size 0 or 1. To run butane is a larger tip ok, or a tip with multiple hole is really better ?
My colleagues have already answered your questions but i'll add a little. I use larger tips when using propane instead of acetylene. Propane specific tips work better than acetylene tips because they are easer to light and stay lit and less likely to blow out when the flame gets close to the work. It is easy to demonstrate this by trying to blow out a frame with your breath. It takes much more force to blow out a mule-port tip flame. A gentle puff will blow out a propane flame coming out of an acetylene tip. Also a multi-port tip (that has a long center flame surrounded by tiny side flames) produces a very sharp flame point that makes it easier to exactly control where i place my heat on a joint.
-do you use a flashback arrestor with the oxy concentrator (low pressure, not that much oČ) ? And for the propane/butane, are all acetylene flashback arrestors fine (enough pressure ?)
no, i do not use a flashback arrestor with an oxygen concentrator. The concentrator does not store enough oxygen to make a bomb or unguided missile like a pressurized oxygen tank.
-what kind of regulator do you use for butane/propane? Is something like this fine, or better have a manometer for higher precision ? basic adjustable regulator
like David in the US,I use a two stage regulator because I like a very steady flow of propane which doesn't require me to adjust my torch when using it like I sometimes have to do with a cheap regulator. In Ukraine (where we build frames for a charity bicycle project) we use a fairly inexpensive propane regulator that works okay just not as nice as a 2 stage regulator. It seems like propane is less fussy about regulating than acetylene.
-do you think brass brazing only will be strong enough for a cargo bike ? I'd like to build a rather lightweigh version wich support about 80kg weight + pilot.!
It has proven to work well for decades.