Quote Originally Posted by polyglot View Post
I did some prints last night and then again tonight; identical exposures (according to my electronic timer) and brand new developer in each case.

The densities are different. Our mains voltage varies a bit and I have a traditional lightbulb enlarger, so my suspicion is that that has caused the exposure error of about 0.15 stops - quite visible in the final print.

So I can see that having some sort of brightness-feedback system as is required for (highly variable) cold heads might also be of some value when printing with tungsten.
Variacs are pretty inexpensive. You already own a voltmeter. If it is day-to-day variation in the line voltage then just set your voltage with the variac at the beginning of each printing session. As you probably know, the automatic regulated power supplies that will keep the voltage fixed will be pretty expensive.