Vintage 1979 Peugeot roadbike rear derailleur change Hello guys, The rear derailleur (6 speed) on my bike (picture attached) shattered all of a sudden while biking uphill after a few months of problems with the chain not fitting immediately on the cassette when I shift gears.

The derailleur's mark is Simplex (made in France). Could you please guide me as to whether I should replace it with a Simplex or can I go with Shimano or something else? what are the characteristics I should be looking for when buying the new derailleur. 

Thank you for getting back to me and for taking into account that I am a first-time bike owner (no experience in parts and repairs)

Cheers!

Nagi
If you have friction shifters, they don't really care what kind of rear derailleur you put on them -- just that they can pull enough cable to move the rear derailleur from the highest gear to the lowest gear. 

If you're not concerned with having a matching vintage set, I'd suggest pop  picking up something like a Shimano Altus or Acera rear derailleur. 

But you may want to have the bike checked out by a bike shop -- when a rear derailleur goes, it may bend the mount on the frame (easily straightened with a tool bike shops have) and if it hits the spokes of the rear wheel, it may de-tension or damage some of them (this can be a bit more annoying).