I have a Tack II. I bought it used from Rio Grande. It works fine
for gold, brass, copper, and not very well for sterling. It seems
that sterling is a very good conductor of electricity, thus the
resistance needed to make the welder work isn't there.
This is a tack welder, and doesn't usually make a good enough
connection to avoid soldering. On occasion, I have welded a
stainless post to the back of a sterling stone set earring and had
it hold. You do need the kind of post with the tiny point on the
connecting end that is made for tack welding.
Rio has demonstrated the use of a TackII with sterling but I can't
make it work reliably.