Quote Originally Posted by Bob F.
Thinking about it, it is possible each pair of lamps are wired in series, in which case only one of a pair may have actually died and that one of each pair is still OK.
Cheers, Bob.
That one is well worth checking. It would be unusual for two bulbs to go at the same time, let alone for the same thing to happen twice. The chances are that two of your old bulbs will still work, unless there is a fairly serious electrical problem that is putting in fairly massive spikes. I don't know the enlarger so don't know how it is wired.