I'm trying to better understand some things about magnetic instant start ballasts for two 75W lamps (regular "Slimline" system). What is the schematic of a 2-lamp 120-volt instant start ballast? The lamps seem to be in series, and the ballast is an autotransformer, so how can a faulty ballast just light one lamp? Going back to our discussion on humidity, I've had issues with my Bonus Line ballasts only lighting one lamp when it is humid. I've also had them fail where they will only light one lamp (albeit dimly) no matter what. Seems like I've lost quite a few to this failure mode but I can't quite figure out what is happening electrically within the ballast to cause this fault.

To assist ignition, one of the lamps has a bleeder impedance (either capacitor or a resistor) connected in parallel. That way the full ballast OCV becomes available for just a single lemp to ignite. Once it does so, the ignited lamp became an effective short circuit compare to the bleeder, forcing the nearly full OCV across the other lamp and so igniting that as well. Then the normal high arc current builds up, overwhelming the bleeder impedance, so both lamps operate in series.
Now when this bleeder component fails short circuit, what you get is just a single lamp connected to the secondary (so working) and the other one shorted out (so not working).

Other spooky looking, but still common failure mode is when the lamp parallel to the bleeder fails to start (EOL, lost vacuum or a bad connection, or even the other lamp having too high voltage drop due to the same reasons). In that case the good lamp will just glow/shimmer dimmly, because the bleeder lets just very limited current through it.

Okay, that makes sense. What causes slimlines to "rumble" when they are cold and how come not all slimlines do it? Is it insufficient voltage available so that lamp arc isn't sustained and lamp has to continually be "restarted" until the tube is warm enough to sustain discharge? The ballasts that are more reliable in cold temperatures are larger and heavier than the ones that don't work in the cold.

Related to rumbling, it seems that lower wattage energy-saver tubes start more easily in the cold than full-wattage tubes. A light that rumbles with 75-watt tubes will do better with 60-watt and better yet with 58-watt tubes. This seems counter intuitive to rapid start where energy-saver tubes seem harder to start.

The lamp ignites with cold cathodes. The high cathode fall (up to 100V) then heats up the cathodes, till they reach emission and so the voltage drop falls to the normal running 10..15V.
Now this warmup may happen at different rate on each side, so for some time one cathode has already good emission so lower drop rhan the other. This then means the total lamp voltage drop is different for each polarity, so forming a rectifier out of the lamp. The resulting DC component then biases the ballast core so it gets into saturation. And a saturating transforme means large current spikes so spiky magnetic leakage field around the ballast, as well as magnetostriction movements of the core, both causing vibrations so the noise.
When both filaments reach emission, the lamp stops rectifying, so the noise ceases.
Although the lamps light instantly, it takes few seconds for the cathodes to fully warmup, so plenty of time for such effects...

Interesting. How hard is it on the lamps and ballast to operate in such a condition? In other words, is it bad to operate slimlines in the cold when they rumble like this?

How come this effect is less pronounced with energy-saving tubes?

Interesting. How hard is it on the lamps and ballast to operate in such a condition? In other words, is it bad to operate slimlines in the cold when they rumble like this?

How come this effect is less pronounced with energy-saving tubes?

For the normal few seconds per start it should be no issue at all. But if it rectifies all the time it is ON, the ballast may be overheating.

The energy savers have lower voltage drop, so pass higher current in the cold cathode mode, so warm up the electrodes quicker.

Thanks!

This is also the case with HID lamps.