[see attachment for schematic]

I bought some Proper Good Stuff for my SOX 90 lamp (and the SON 50s). I only just found out, that it appears to run the lamp with a capacitor sort-of in series with the lamp.

What's its purpose? I bought the ballast assuming the cap would just be for power factor correction and therefore optional in a home environment - i don't have one in exactly that value, and it specifies +- 4 procent tolerance which is pretty extreme

I'm tempted to just slap in a common 10µF capacitor with a high enough voltage rating. It'll be fine, even when overrun a bit, i'll never wear it out in my life time i think. But i also still wanna know what it exactly does here.

Update!

Took a bit of sleuthing in brochures and datasheets to find the exactly right model, but it's not a simple choke ballast as expected, but a somewhat unusual (in Europe) constant wattage ballast!

It seems to 'just' be current limiting, but any deeper explanation on how this thing works is welcome.

CWA ballast uses the capacitor as their ballasting element.

Aaaaand found a capacitor. Turns out, i got lucky - the mercury vapor gear trays of the lanterns that are getting removed at my place of work contain 9,6µF power factor compensation capacitors (this time really just PFC).

So i took one out because those MV ballasts will run fine without.

Remember that the series capacitor in leading and CWA style circuit generally works at higher than the line voltage (450V rating typically) and the capacitor borrowed from PFC part (usually 250V rated) may break down.

Yeeeeeesh my room is YELLOW now. That thing puts out a MASSIVE amount of light.

I put a meter across the cap, RMS voltage started at 265v slowly dropping to 210 at full brightness, with 230v mains input. The cap is rated at 250v 60hz, or 280v at 50hz. So a higher voltage one would be best for proper durability, but this one will also likely last the rest of my life since it's too bright to run very often.

It will do very nicely for the sodium-related physics demos at work.

Update!

Took a bit of sleuthing in brochures and datasheets to find the exactly right model, but it's not a simple choke ballast as expected, but a somewhat unusual (in Europe) constant wattage ballast!

It seems to 'just' be current limiting, but any deeper explanation on how this thing works is welcome.

Roughly, all that wickedly connected autotransformer circuits in the end are electrically equivalent to a series LC tank in circuit with a lamp. Current stabilization arises from ferroresonance phenomenon. When current increases, it starts to drive a magnetic core or some part of it (magnetic shunt) into saturation. That decreases effective inductance, and detunes series LC tank circuit out of resonance, increasing the impedance.

Usual LC leading fluorescent ballasts are displaying this behavior to some degree, too!

Five years ago I bought the same BSX ballasts and still have the same problem like you with the capacitors value. I treated that problem by adding an 8.0uF motor run capacitor in combination with a 1.5uF one in parallel which gives 9.5uF in total. The voltage is also not a problem because these caps are always rated at least for 450 or even better 475V. The lamp runs fine and ballasts cool.