Try to get hold of a 6603 mech, I ran one on my inbred 29er for a good few years, using just such a chainring combo.
Thanks tiermat, if I can get hold of a set of 6603, will definitely give it a try.
The DA (and Ultegra) triple mechs work in a 12 or 13 tooth difference between outer and middle. Thus the inner cage side hits the centre ring if the mech is positioned as specified above the outer ring. A 105 triple mech will work with the 10 tooth difference I believe. Check the mech specs on the Shimano tech docs.
Thanks for pointing the direction rafletcher!!!
I was trying to find the spec docs right after read the replies from here. I found FD-7803 spec from 2007 listed as Top gear teeth -> 52T, capacity -> 22T, and chianstay angle-> 63~66 degrees. FD-6603 also share the same spec on top gear, capacity, and chain stay angles. I didn't saw the outer and middle chainring capacity in road series spec column, but I do saw it listed as "Top-Middle Min. Capacity" from ATB(MTB) section. And the 105 (5603) did has different spec on Top gear teeth (50T) and capacity (20T) while chainstay angles are the same.
There's an Ultegra 6603 triple mech for sale here: https://www.bankruptbikeparts.co.uk/shimano-ultegra-fd-6603-front-gear-mech-derailleur-bottom-pull-31-8mm-50-bbp2210859
Are you using STIs or bar end/down tube shifters? If you're not using STIs, I'd be tempted to just go for a MTB front mech, gives you more flexibility of touring-suitable chain ring sizes.
On my tourer I'm now running 46/34/24 with an XT front mech, the only problem I have is when on 34/11 I get some chain rub against the pins of the outer Chinook chain ring, otherwise all good.
XT front mechs are discontinued, though and seem quite hard to find, Deore front mechs are still readily available, though then you're going down in quality a bit from Ultegra or XT
Thanks for the "shopping guide" on 6603 Oxford_Guy. ;p
I am planing on (and already have a pair of SL-7900 in hand) use downtube shifter on this bike, and do considering to switch to MTB series of front mech to save some time/money on get a new crankset. the thing is with my limited knowledge( experience) I am troubled with choosing the suitable one.
I am fairly new to the cycling world with very little experience/knowledge. still digging, hopefully I can finish the project alright. Thanks for the suggestions and direction pointing! and sorry for not having visual guide on the specs docs, cause I don't know how to put the picture here via computer. I will try to put them on via mobile phone later today. Maybe also the pictures of the parts I already get.
Cheers!!