here is a grow faq written by NIMBLY from O.G.

Pyrex baking tube

(NIMBY) "Using a Pyrex (borosilicate glass) tube obtained from a glass blowing supply house or using a "Bake a Round" (eBay had a dozen for sale the last time I checked) one utilizes either one or two (pictured) 4" starter collars instead of the 5" to 4" reducer collars. They are 14" long and 3.75" in diameter."



"I stretched the aluminum ducting out and measured 16". I then snipped the metal "ribs" and cut the ducting open. The glass tube will now just drop into the long run of ducting. The electrial wires run to the remote ballast through the intake part of the duct (exhaust could also be used depending upon the location of the ballast). I measured 2" from each side of my original cut and snipped the metal ribs again but this time didn't cut the aluminum foil. This allows me to open the ducting up like a "wing"."



A couple of wraps of pipe wrap sealed with foil tape on each end you want to put a collar on should be used to keep from biting the metal directly into glass with the hose clamp (pictured). The socket is mounted inside the tube with pipe strapping just as in the hurricane style fixture. It can either be "clipped" and taped over the edge of the glass or better, pop riveted to the inside of the starter collar.

Simply stick the glass inside the end of the starter collar an inch or so past the bottom of the tabs to measure how far in to drill two holes 180 degrees apart, then use two pop rivets to attach the strapping

A note about pipe strapping: don't get the thin wimpy stuff. Get the thicker heavy-duty strapping. The heavy stuff is still relatively easy to bend but holds it's shape better and will hold the bulb and socket straight without sagging. At Home Depot they even have some copper pipe strapping (also known as “pipe tape" or “pipe hanger”) that is quite stiff.

Ventilation Performance
There are many different ventilation options available, since standard household ducting is used in the construction of the fixture. For those folks with bigger boxes or rooms, ducting in and out, "inline" duct fans are probably the best option.

For my little NewGanjaBoy-style setup, using the Hurricane fixture as part of the ventilation system of my box, a 115cfm computer fan does the trick. As for actual performance specs for different blowers/fans and light wattages, I'm afraid you'll have to experiment. Here's mine just to give an example:

Box:
-NewGanjaBoy-style three chambered box
-4 20w flouros in the mother chamber
-150w security HPS in the flowering chamber in original metal fixture with holes drilled in the top

Ventilation before Cool Tube installed:
-115cfm fan exhausting box
-4"x8" intake port in the bottom of the veg chamber
-Two 2' runs of 1.5" PVC pulling air through the wall between veg and flowering chambers
-Two 1' runs of 1.5" PVC pulling air from over the HPS fixture into the utility room where it's exhausting out the back.

Ventilation after Cool Tube installed:
-Two PVC runs between flowering chamber and utility room replaced with Hurricane Cool tube fixture
-ballast moved to utility room and housed in the original security light casing
-everything else is the same

Temps before Cool Tube mod:
Ambient temp: 80°F
Flowering chamber 1 hour after HPS fires up: 95°F (in direct light)
Flowering chamber 6-12 hours after HPS fires up: 100-105+°F (ouch!!)

Temps after Cool Tube Mod:
Ambient temp: 80°F
Flowering chamber 1 hour after HPS fires up: 85°F (in direct light)
Flowering chamber 6-12 hours after HPS fires up: 90°F (in direct light)

SAFETY NOTICE:
Please note that the wire to the bulb base must be a high temp fiberglass type, or the heat will eat up the wire and cause a running short. The thermal tape is a fiberglass electrical tape from most hardware stores. High temp fiberglass wrapped wire is available at any hardware or electrical store. It is imperative that you use it, as a smoking ballast is a real bummer to relight.

Last modified: 03:57 - Feb 21, 2003 Quicklink: http://overgrow.com/growfaq/1432
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faq:1432 "How do I build my own Cool Tube?"