I'm seeing my exposure times rise for exposing enlarging paper, Ilford Multigrade in this case. My times have risen at least 25% (e.g. an 8 second exposure now requires a 10 second exposure for the same results). I'm still working on the same batch of 500 sheets that I've started with last year so I'm thinking that the paper is slowing due to age. It's also possible the bulb is weakening due to use (I once left it on accidently over night).
Any other thoughts as to why?
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"The secret to life is to keep your mind full and your bowels empty. Unfortunately, the converse is true for most people."
First, my Philips PCS130/PCS150, which uses three MR11 halogen bulbs (for red, green, and blue light), blew a bulb a while back. I replaced it with a spare bulb that came with the enlarger, but I found that I needed to radically up the output from the replacement bulb. When I looked more closely, I found that the replacement bulb was cloudy, as Richard notes. I don't know if the bulb was defective new or if it had been used, deteriorated, and set aside for emergency use only by the previous owner. In any event, it's worth checking the bulb for obvious discoloration/darkening.
Second, I mostly do 8x10-inch and smaller prints, but occasionally I do larger, so I keep some 11x14 sheets in my freezer. After about two years, I pulled these out to make some prints, but I did some test strips on newer 8x10 sheets of the same brand and type (Foma Variant 311 VC RC). When I went to make the final enlargement, I found that it was much lighter than my test strips on the same setting. I suppose this could have been due to emulsion-to-emulsion differences, but my suspicion is that it was the age of the paper -- even cold-stored, it seems to have lost speed over a 2-year period. As I recall, I needed to roughly double the exposure to get matching results.
What about unstable line voltage? Do you have equipment kicking on and off on the same circuit as the enlarger, such as AC? This would create problems with color printing, but I think a voltage fluctuation would be in the range of 5-10%. I don't know if that's enough to cause a 20% exposure error. I'm guessing.
Paul
__________________ "Pictures are not incidental frills to a text; they are essences of our distinctive way of knowing." S. J. Gould
Incandescent bulbs used for photo printing usually only have about 3.5 hours of total life, when used normally i.e. the bulb turning on and off (which increases its degradation). I used to use a current timer that I took out of an old server back in the late 80's and put it in between the timer and bulb so I could keep track of the age/life of the bulb. Going from an old bulb to a new one gave me a 1 grade of contrast and 1 stop speed. This comes in handy when doing series of images and you don't want to change in the middle of it, and thus change the look of the series.
Paper Slowing? 
Linear Mode
