Hi Bob and group
Regarding the matter of cooling a pair of 4-400s with a forced air system.
Rich Measures is on target. Let me amplify on his points in reverse order.
1) The performance of axial type fans fall apart when delivered against a
back pressure. Muffin fans that deliver 100CFM in free air drop to less
than 10 CFM at the rated back pressure of the SK-410/406 system .2 inches
if my memory serves me correct. Tarzan axial fans 250 CFM (free air) will
work but they are just as big and just as noisey if not noisier than
Squirrel Cage fans.
2) Heathkit got slick in their approach to cooling the SB-220. Their
engineering people realized that passing air over the glass envelope was
NOT the critical factor in cooling the tube. Rather it is keeping the Plate
and Filament Seals below their rated temperature. 225 degrees C in this
case. So what they did was to make sure that there is adequate air flow at
these points. They approached this by first postioning the tubes so that
the filament pins are facing one another. Then blowing air laterally on
the tubes. They cut out a piece of the chassis and used a fan with a large
enough blade so that air passed across both the Plate and Filament
connections.
Getting back to your problem Bob you can tackle it one of 2 ways.
Unfortunately it sounds like your are half way in either direction.
If you want to go with the forced air system you will have to scrap the
muffin fan for something in the way of a squirrel cage. (3 inch @ 3200 RPM)
If you want to blow air directly on the the Muffin fan does not have a
blade large enough to hit all of the areas involved. You can use it for the
Plate Seals and the tube itself but you will have to find another small fan
to cool the filament lugs. Also if you choose to go this route don't use
the SK-410 socket as it shields lateral air flow from the tube. The Ceramic
275 socket is fairly cheap and leaves the pins exposed.
Have fun
Bob
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