Thought of this on holidays. I had problems with a front
derailleur and could not get enough tension on it to get it
to work properly, even using barrel adjuster, as I did not
have a big pliers.
I thought and thought and then...
I removed the cable stop where the left downtube changer
used to be in older bikes, adjust the cable and then pulled
the stop back into position and fixed it. I was then able to
slowly let the cable out by trial and error until I got the
changer right.
A good trick. I've never seen anyone do it, although some
must have thought of it years ago.