ml wrote:
i've build a 80 center fed dipole, cut for 80, to use multiband--
fed via coax.

It is not a good idea to use a coax-fed 80m CFD on the other bands
because of losses due to SWR in the coax. Your balun problems are the
least of your worries. If you feed the 80m CFD with ladder-line, you
could use it on the other bands without suffering massive SWR losses.
For that configuration, all one needs is a 1:1 choke at the tuner. Take
a look at http://www.qsl.net/w5dxp/notuner.htm for some ideas on your
antenna.
--
73, Cecil, W5DXP