Other than Aristo, can anyone reccomend a cold light source for my durst 138S?
I am currently using an Ilford Multigrade 500 head and ideally would like to modify this into a cold light source. The head and complete ilford system is quite handy and if possible, I'd like to keep the basic set up, with the cold light being the only modification.
Another question. Are all color heads cold light sources?
although I can't answer your main question, I'm not sure if you really want to adapt a cold light source to a Multigrade 500 head. A cold light source is one that does not have a conventional bulb (and thus produces far less heat - the reason why it is called a cold light source). It is usually a discharge lamp, similar to a neon lamp. And I know of no colour head that uses a cold light source.
People usually use cold light sources as an alternative to a condensor on the one side and a diffuse color head on the other side. A cold light source does have the same diffuse soft light as a color head but is simpler to handle and does not require active ventilation. And a cold light source is usually much more efficient (it is brighter and does emit more blue light, to which paper is most senstive). The drawbacks are a reduction of dodging and burning times and/or lower and worse perfoming apertures on the lens.
You also have to take into account, that the different spectral composition of a cold light source changes paper speeds for Multigrade as well. Grades 4 and 5 will take double exposure time with normal light, because the amount of blue light needed to expose the "hard" emulsion part on the paper is low with a normal bulb. This is no longer true for a cold light source. It is not a serious problem, but it does require some adjustments and you'll have to become aquainted with it again.
BTW: the usual cold light setup for a Laborator 138 is the use of LACOLI/LACORTA, but this will throw out your Multigrade head.
FWIW, the Ilford Multigrade 500H is not a color head, but rather a VC head, using 2 halogen light sources that are reflected through a diffused mixing box. The VC filtration is then controlled on the timer.
After some research on the net, I've come to the conclusion that the only alternative would be to use the Aristo. They have a model specifically made for the 138, and even offer the power source in 220V, which also offers a similar VC filter control as the Ilford System.