You must have a little different one than I do..
I would attach the large spring, the large cap and the C clip on the end of the assembly before I tried to put the smaller parts together. it is easier that way.
As far as the bias cell is concerned. you can’t use a 3 volt battery. it will give you a terrible hum..
The bias cell puts a bit of negative voltage on the grid of the tube. Here are some photos of the battery holder I use.. It is a battery holder for a 3 volt battery, but the 1.5 volt battery I use is too small, so I use a hot glue gun to keep the 1.5 vole battery in place.
Make sure you get the polarity of the bias cell correct. you can buy these 1.5 volt batteries at almost any dollar store. They are watch batteries.
I am sorry I can’t help you any more with the tuning assembly.
Dan in Calgary.