Too much voltage to headlights?

Any progress on this? I just changed the headlamps on my 70 dart for the 3rd time since installing a relay harness. When it happens both low beams go out at the same time. Charge voltage is good, a bit low at idle but mid 14's at cruising speed. Whats the absolute max voltage these sealed beams can handle before they blow? I imagine it's a bit north of 15v!