2 Century lamps that work well without heat ring?

[Deviant Grasshopper]

I recently purchased 2 matching century lamps. Neither have a heat ring - one flows perfectly (albeit a bit slow to warm up compared to my newer aristocrat) and the other is actually a little hot and needs a dimmer.

Is this common? I made a makeshift heat ring out of aluminum foil and the silver duct tape but it made them run way too hot (too many small bubbles rather than 'flowing' like they should). Maybe this is why they're removable - as the lava ages it flows easier?