Wheels keep killing tubes

[Kamoshika]

Can anyone offer any suggestions as to what might cause a set of wheels to repeatedly have tubes failing at the base of the valve? I replaced the wheels on my wife's old MTB a couple of years ago as the brake track was wearing thin. There was limited choice of 26" wheels available. I swapped the tyres and tubes over from the old wheels, but after a couple of rides one of the tubes failed at the base of the valve stem. At that stage I realised I had inadvertently bought wheels with Schraeder drilled rims and, not noticing, had put presta tubes in, so assumed that was the cause. We had spare presta tubes, so I got some little plastic inserts to downsize the hole. Again, a short time later the tube failed, on the other wheel this time. Having run out of spares I then bought some schraeder tubes and fitted them instead, but same problem - tubes failed at the valve stem. There was a slight burr around one of the valve holes, so I've filed that down smooth in case that was the cause, but again the tube has failed. It's affecting both wheels. They're both fitted with good quality rim tape. I can't see any obvious cause, and I'm out of ideas for things to try. Can anyone offer any suggestions before I give up and buy different wheels?

[LittleWheelsandBig]

There are couple of possibilities, though you have already covered the easy ones (correct valve type for hole, burred valve hole).

An undersized tube will have to stretch quite a bit to fill the tyre. This sort of thing is generally ok but the valve causes additional localised strain which can cause premature failure at the base of the valve, at the side of the valve.

If the tube is not correctly positioned circumferentially, then the valve stem can be canted i.e. not pointed towards the hub. This situation can again cause localised failure at the valve, in line with the rim this time. Some folk straighten a canted valve by tightening the valve nut, which just makes the tube fail more rapidly.

If the valve nut itself is overtightened, you can overstress the rubber around the valve causing, you guessed it...

I don't use a valve nut except for tubeless valves. If you are the same and use a frame pump with a rigid thumb-lock, then a poor pump technique can overstress the base of the valve but that usually fails the valve itself, rather than the tube around the valve.

[Kamoshika]

Thanks for the suggestions. Will double check the tube sizes, but 99% sure I've bought the correct size.
The last set of tubes had rubber up the side of the stems, so no valve nuts, but I don't think it's that - I've always been cautious of overtightening them. The last tube that failed was at the side of the stem. I'm not sure about the others, but I've still got them in the garage, so will check. Similarly, I've always been careful to make sure the valve is sitting straight in the hole, so I don't think it's that either.

[Zed43]

Did you buy a batch of tubes, and are those failing? Earlier this year I had the same thing happen to me with a 26" wheel that I have been riding for 5+ years without issue and then suddenly a flurry of failures as you describe. From the top of my head this was a batch of Schwalbe SV12 tubes bought from bike24. Knock on wood but I've been ok in the past few months.

[ElyDave]

Wire-beaded tyre?  I had a case where there was a miniscule piece of wire protruding inwards from the bead that was puncturing the tube.  Because I was lining the tyre up with the valve, it kept failing at the same spot.

[Dickyelsdon]

If you refitted old tires they could be stretched/aged and so a bit loose if the new rims are slightly undersized (talking 1/10th of mm here). Tires can slip on new shiny rims, especially low pressure mtb.
Combine everything and tires slip tearing valves out of tubes.

Mark the tyre and rim to see if there is any relative slip after a few rides.

[This obviously assumes the bike is being ridden, and not just killing tubes sat in the garage.]