I often solder after applying my gold and also occasionally run into
the bubbling problem. Although, the difference is that I have been
able to solve the problem by burnishing the gold back down. I have a
couple of suggestions that may help:
How thick is your gold? I use about 0.03mm. It is possible
that your gold is too thick making it difficult to burnish back down
once it bubbles.
Are your surfaces clean? Dirty surfaces may cause poor bonding
between the two metals.
I do my burnishing on a hotplate. The kind with the open coils.
When you have to smooth out your bubbles, after the tube is soldered
on, could you not use a hotplate and position the tube part in the
spaces between the hot plate coils.
You might consider texturing your fine silver i.e. by using roll
printing. I find that when I use textured silver, the gold seems to
bond better to the silver. Could be because the textured silver has
more surface area. Hope these suggestions are helpful.