A note about opal bulbs in general. Try, if possible, to get a real GE or Philips. Many times if you're ordering a PH-xxx bulb on a website the picture will show a GE bulb but what you'll actually get is a piece of crap "Eiko" thing from China or wheverever. These are notorious for having very non-uniform opal coatings, particularly on the top of the bulb - concentric circles, spirals, wavy patterns etc of higher/lower density. With a condenser enlarger, these irregularities WILL show up on the baseboard. I experienced lots of problems with this in my old Omega B66. I still have a pile of crap bulbs I couldn't use. Don't cheap out on the bulb.
I use those crap bulbs and never had anything show up on my prints.... Maybe there was/is a bad batch out there but mine has been fine. I have a stock of those. Can you buy "name brand" bulbs made for enlargers anymore??
Type, size and construction of the bulb can atter depending on the construction design of the enlarger.
As you use an enlarger with condensers, you should have a bulb large enough to light the top condenser totally and of enough power to get a decent light level on the baseboard.
Try to measure the evenness of the lighting on the baseboard to check if the bulb is large enough to cover the top condenser : put a grade 5 paper on the baseboard, and to a test strip in order to get a pale grey on the center spot of the baseboard.
Then expose a full sheet (the largest the better) at the same setting. As it is a grade 5 paper, any light ouptut variation will show.
So you'll know.
Often you can adjust the relative position of the glass of the bulb regarding the condensers. It is not clearly described in the user manual but...
My Durst enlarger was designed for 4" dia bulbs which are no longer made. In order to even the light on the baseboard, I am forced to use a very powerfull lamp and a frosted glass betwenn the bulb and condenser. I have less than a 1/3 stop difference between the center and the corners of the baseboad....
If you have the L138 you might try a G40 lamp.
There is a thread where someone tested the G40 against the real Durst bulb, and it matched fairly well. And best of all they are easily available, and very inexpensive compared to the Durst/Thorn bulbs.
I plan to use a 60 or 100w G30 in my L1000, once I get it setup.