Pretty much every intermediate (that is only an intermediate) can accept productivity modules, but not the HV regulator. I understand if you did this on purpose, because all that productivity stacking does have the potential to get a bit ridiculous, but I would personally prefer the consistency. Add a startup setting for it maybe?

The HV power regulator recipe already accepts productivity modules (checked with a few different mod lists, just in case). If it does not in your game, it is probably because another mod changed it.

If you have a mod that adds new productivity modules, that is a likely culprit - the recipe restrictions are stored in the module prototypes themselves, so defining modules in unusual ways can bypass other mods adding recipes to the list. If other modded intermediates are still allowed though, it's probably something else.

Hi, I think I got confused between the electric furnace an the hv regulator, the ingredient lists are very similar and I was setting them both up at the same time for production science. Sorry!

Lovely mods btw