I have a Nikon D80, and I use it with the Tamron 70-300 and an old 2x teleconverter. When I mount both of them on my camera in Manual mode, I can change the shutter speed but I can't set the aperture.
Is there any trick to this?
If your converter is too old, it probably misses the contacts to transmit the aperture setting to the lense.
Because the missing electronic contacts between the lense and the body, I think the only mode which works is the M mode: according to this link your setup should be equivalent to the "non-CPU lenses" lines.
This is assuming you do not have an aperture ring in your lense, because the D80 does not fully support the control of the aperture with the ring with AF nikon lenses (CPU lenses) and it should be blocked to the smallest aperture in order to use the front weel (electronic command).
For older manual lenses, the D80 can use them in Manual mode only and you have to turn the aperture ring to choose it (front weel will not work). But in that setup, you loose the metering (footnote 11 in the above link).
Summarized: if your tamron lense have no aperture ring, then your converter (or any extension you put between) must have the electronic contacts (connectors). If you have the aperture ring, you have to use it directly from the lense but no light measure is possible in that case (behave like the camera have an AI(s) lense.