I really got a kick out of this post... Increase the life of a bulb that normally lasts a good 15 years anyway is rediculous.
Me 4x5 beseler has been burning bright for the past 30 years and i do use it for at least 25 hours per week. I do have 3 extra bulbs as back up i bought several years ago thinking these bulbs will become a problem to find once digital gets a foot hold.... Wrong!
So perhaps a better solution might be purchasing extras while they are available n cheap.... Base your needs on your average life of the bulb and your expected life span.
Hahahahaha.... Oh btw kids... LEDs will make a good substitute if not a better solution as well as upgrading your enlarger with a technology with a much better longevity track record.
Oh i also bought 2 extra cold lamp bulbs n a spare head i will ever use in my life time... Yeah my garage is full of spares... I wish i could find a spare heart, kidneys, liver, knees... Etc instead of all this crap my kids will toss in the garbage when i'm dead.