That's a great plan if bulbs are still available for your enlarger. But there is at least one system, the Durst 138S that I have, where the bulbs are no longer available. It was in this context that I posted the idea to extend the life of the few existing bulbs. Given that new bulbs go for over a $100 it makes sense to me to save the few that you can find. I never intended people to buy a variac to save a bulb that you can still but for $5.
Originally Posted by paul ron
For the Durst you have a few options. Durst Pro sells a replacement lamp system for over $1000. You can scour ebay and try to buy the official bulbs if they show up (about two a year it seems). You can use the PH303 bulbs that work pretty well. No idea how long they will be available, so buy a few spares. They are also 500 watts, so the variac can help reduce the brightness. You can buy a kit from Glenview that repurposes an existing bulb. One came with my latest enlarger, and I can confirm it works fairly well. Or you can do what I finally did and build an LED system.
So as a couple of you have pointed out it's a bit silly to worry about for most cases, but I'm sure some people will find the ability to extend the life of rare bulb to be valuable. Even if you don't intend to implement the idea, I still think it's interesting how a small change in voltage can change the expected life of a lamp.