From experience a cooltube is great in the right space.
The fact I can bring my hps within "4-"5 from the canopy is great. Making the light intensity on the canopy higher. Along with establishing an engineered heat safety zone above the canopy.
The bulk of the heat created off the bulb is carried away removing it from the grow space.
People suggest cooling your bulb reduces spectrum. What happens when your environment averages °86-°87 and the heat from your tent is the air used to carry away the heat off the bulb. This makes it so the spectrum is less affected.
The only cons I see,
The light spread isn't very consistent. But is heavy focused in a column the shape and direction of the cool tube glass.
The glass blocking some of the uv light. But it doesn't seem to cause me any problems. Hps bulbs don't emitt much anyways.
Hey bro the beast is grown under a cooltube :)
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My tent is very small and spec'd for 250 watt hps. So overdriving the space with a 400 watt hid required me to remove as much heat as possible.
The cooltube was the smallest air cooled fixture I could fit in my tent
The small tent helps reduce the uneven light spread of the cooltube via the reflective walls.
It has its places to be used for sure.