Practical limits and soldering tubes?

First off, how does one solder copper tube? I need more the 50' and I only
have 50' rolls available here. I can get another roll, but what is the
best way to connect them? Please include a quick tutorial on the soldering
process too, I've heard "sweat soldering" mentioned on here before, but
have no idea what it means. ;)

I have been playing with numbers in WinTesla. With info from the group and
from there it seems I can add very big toploads if I increase my primary
size to compensate leaving the caps and the power source alone. Of course,
with the math I could get some very large numbers. Can anyone help me
determine the practical limit to this? Quick coil specs are in order.

15KV 60 mA Neon
.01 mfd cap
6" dia. 24" secondary
8" flex duct topload
Series static gap

Of course, at some point the power source will not be able to deliver the
required power. However, for the system described, what is a good place to
start? How about getting another transformer in the mix and running at 120