I have a Zone VI cold light head from the late 1970s (4inch x 5inch). It has no transformer, the tube just plugs directly into the 110volt mains. I have a slightly later one for a 2x3 Beseler and it has a transformer. I have no idea why the difference.
Originally Posted by NedL
I used a Zone VI Stabilizer, then later a Zone VI Compensating Timer, which has the advantage of being more efficient and giving shorter times. As the tube warmed up the "ticks" of the timer, which represented a nominal "second" of exposure sped up dramatically as the tube brightened. I never actually tried enlarging without either the compensating timer or stabilizer, but the comments above suggest that a reasonably stable light would be obtained by following Ned's advice above.
I like the cold light head. My prints improved when I started using it. Not that it is "better" but because my negs were too contrasty for my condenser head, as a result of following the manufacturers' instructions for developing (to a highish contrast, maybe to justify advertised film speed).
I don't know if VC filters will work well with your lamp. Split grade printing (Ned's suggestion) may well be the answer, combined with negatives of ideal contrast to put you in the middle of the range. New tubes with a wider spectrum are available from the company that took over Aristo's business, but they are expensive.* Worth it in my opinion, but see how you go with the current tube.
Incidentally, in the 1970s I bought two spare tubes from Zone VI but never needed them. They do last for ages. The early ones looked a bit on the blue side.
*For new "Aristo" tubes see this post. They also make the wide spectrum tubes for single tube units so that VC filters work well.