Has anyone tried using the referenced cold light heads without a compensating timer?
I am curious just how much variation can be expected due to light tube temp fluctuations. Does anyone have first-hand experience using the light heads directly without Zone VI modifications?
I've used the Zone VI head without the compensating timer. It has two electrical connections - and I don't know if I remember how they work exactly, but one is powered all the time to keep stability in some manner, and the other is switched. When I used it, I would switch on the light with a card between the lens and paper, wait a few seconds to give the light a chance to stabilize, and then make the exposure.
I think the main use for the compensating timer was to ensure consistent results through long print runs. I never made more than two or three prints at a time, so I never saw the variation that longer runs can exhibit.
Of course, the timer also had a provision for dry down compensation.
Unless your doing large numbers of prints, I don't think you will miss the absence of the timer.
I've used cold light heads for almost 25 years. Before VC paper improved I used a De Vere with graded papers and of course I had no compensating timer. My experience then was that unless I kept the enlarger at a consistent temperature the final print did not match the test strip. I overcame that problem by leaving the enlarger on as I developed the test strip and then switched it off to load the paper to make the print and it did work although it was some what crude.
I now have two Zone VI VC enlargers with RH Designs f Stop Timers and cannot fault either. The compensating timer takes care of the fluctuations in the illumination and has a number of other excellent additions like, 2 channels for split grade printing, the ability to change the f stop setting fron 1/2 stop through 1/3, 1/4, 1/6, 1/12 and 1/24th stop increments as well as a very good dry down feature.
I have also seen the new version of the Zone VI enlarger when I visited Richard Newman a friend who works for Calumet in California. The light source is LED and it is very good and I think it is now available.
The first cold light head I had was an Aristo and it had a heater in it. I would turn it on about 15 minutes prior to starting to print. I don't think I had an issue with that head. I might not have known enough in those days either. I have used Zone VI heads where it had a green and red light on it. when the green light was on you could print the photo and when the red light was on you needed to hit the print button and turn on the light for a second or two. The Aristo VCL 4500 that I now have is hooked to a Metrolux II timer that has a probe that keeps the time accurate relative to the amount of light that is used. A closed loop if you will. It is nice but not technically necessary.
I used to use a Zone VI head with the Zone VI compensating timer. The length of the units of exposure vary in real time with the light intensity. An audible "tick" sounds after each unit, about one second. As the lamp heats up, the ticking accelerates markedly, indicating a very significant output increase during the exposure.
The heater element in those units is always on, and goes some way to 1. increase light output and 2. slightly reduce the difference in output between cold and hot. The Aristo VCL4500 (and maybe other models) has a thermostat on the heater, so some regulation is built in. These "cold light" heads can get pretty warm. My Aristo VCL4500 is 80degC (176degF) inside at the end of a longish exposure.
I think I remember reading that the later (current) Aristo tubes are less variable with temperature increases, but I don't remember where I read that. The early Zone VI single tube models can be fitted with a current Aristo tube that is better for VC filter use (wider spectrum- the old Zone VI tubes were rather blue because that's all you need for graded papers)
The RH Designs Stop Clock Vario has the kind of accurate feedback correction referred to above by Les.