Apologies for the dullness of this post!
I’ve had the problem of inner tubes tearing around the valve once before and cured it by replacing the rim-tape. It looked to me like the rim-tape had slipped, exposing the pointy metal valve-hole to make a mess of my tubes.
Now it’s happening again (twice within about a mile) and I can’t figure out why. The valve-hole looks fine to me, but I bodged a solution with my last tube and the remainder of a busted one: cut out a section of dead tube, poke a hole in it with knife, feed the valve from new tube through this to protect the valve area. That held up for a further 4 hours of riding. There must be a better answer and a reason for it happening rather than just treating the symptoms… Any ideas?
Oh, yeah… presta tubes, Mavic 717 rims, Conti Gravity 2.3 tyres, around 35 psi