It might be nice to putthe ferroresonant downstrem of the timer, from a sound perspective, but the light output could be non-linear to time for shorter expsoures.
Ferroresonant units stabilise output by building and storing a surplus of energy in the form of an extra winding that will keep the magnetic core that powers the output winding at a constant output level over short perionds of time. It does take time to build up that field, and while doing so, the enrgy is not available to go out as ouput power.
Kind of like adding extra mass tuned to adjust the resonant freqency of the suspension of a motor vehicle. Makes for a smoother ride, but the extra mass means moderatly slower acceleration from a stop.
Also when you stop the power, the ferroresonant dumps the extra energy stored in it, but not necessarily in the same voltage profile that it absorbed it in at at the start of the process.
So the light may have a differnet intensity/colur at the end as extra field collapses.
But surely as it's AC, we are only talking about one cycle worth of time?
Depends on Q.
My understanding is that the regulatory behaviour is from saturation (i.e. the core is grossly undersized) and that the extra resonant winding exists just to reduce additional harmonic content, i.e. prevent it producing just square waves. So the quantity of energy stored by the resonant coil would represent probably a cycle or two's worth at nominal load. But it could well be many cycles, depending on the damping factor of that resonant system.
A Zone VI Compensating Enlarging Timer paired with a Zone VI Compensating Developing Timer.
Test strips and prints are freaking identical. Every single time. Even weekends apart, if I use fresh developer from the same batch.
So consistent it's unnerving.
For perspective, I've even run tests on the developing timer, varying a water bath by 0.1F against my standard mercury thermometer. Moving between 67.9F — 68.0F — 68.1F I've observed the addition or subtraction of ~2 seconds per minute. Rezero and it goes away. Move in the opposite direction and it follows exactly as expected. Back and forth. Consistent as hell.
If the tones are wrong, I can't point fingers. I know it's my fault every time.
I would expect RH Designs more modern hardware might be even better.
Another inexpensive solution is a second-hand baseboard color meter. I got a free Melico color meter a while back. The 'white' channel nulls with a good sized wide swinging needle that is easy to read and center.