Bulbs for some enlargers are getting hard to find. The opal bulbs for the Durst 138S condenser heads have been out of production for many years. Occasionally new ones still turn up, but they are expensive and hard to find. If you have a bulb you probably hope it won’t die on you until you can find, and maybe afford, a replacement. This was my situation when I got my 138S. I had one bulb that I thought was original (turns out it isn’t, but it works well). When I eventually acquired a few of the original Durst Atlas Thorn bulbs I was hesitant to use them for fear of burning them out.
I had known for a while that a bulb run through a dimmer will last longer, but I wasn’t sure how much longer. Still I thought it would be prudent to try to extend the life, so I bought a Variac off of ebay. These are transformers that take your line voltage and bump it up or down in an adjustable fashion. Mine will take 120V and transform it to 0 to 130V. It plugs into the wall and has two regular sockets on it that you can plug your enlarger into. A big knob controls the output voltage. The scale on mine doesn’t exactly match the output voltage, but it’s very repeatable, so I just marked a few positions with the actual voltage. I recommend testing the voltage with a multimeter (only if you know how to use one safely with AC line voltages) as you don’t want to accidentally run the bulb at more than its rated voltage.
I found with the 500 Watt bulb that each 10V drop decreased the light output by 1/2 a stop. I settled on 110V as the maximum that I would run the bulb at to extend its life. For smaller prints I generally find the 500 Watt bulb too bright even at 110V, so I run it at 70 to 90V which gets me one to two extra stops of time for easier dodging and burning.
What I haven’t known was how much this could extend the life span of the bulb. However I just found the May/June 2000 issue of Photo Techniques magazine (I got a whole stack of these with the enlarger) which surprisingly has an article, “Optimize Enlarger Light Intensity” by Conrad R. Hoffmann, on reducing the light output with a Variac. In the notes section he gives the formula to calculate how much the lamp life is affected by the voltage. The formula is (Design Voltage / Actual Volatge)^12. Yes that’s the 12th power, so a minor voltage change can have a huge effect. For example running the lamp at 110V will give a 2.84 longer lamp life. That right there more than pays for the cost of the Variac, which ran about $100 and is what I see the bulbs going for recently. Running at 70V will increase the life by 644 times. That may mean you no longer need to worry about burning it out.
One thing to note is that the color temperature of the light drops as the voltage drops. Mine went from 2800K at 110V to 2300K at 70V. This will change the contrast some with VC paper. I think I lost about 1/2 a grade when dropping to 70V. I suppose you can use this to get intermediate grades if your filters aren’t fine enough for you.