>Steve is right. The turbulent air from the outlet of a fan makes a
difference to the cooling efficiency.
If you follow a thin streamer of smoke through a typical flat-pack
the streamer holds together as the air flows smoothly into the fan -
on the other side it vanishes, completely mixed in by the turbulence.
thin thread of cotton held in the air-stream shows the same effect.
With an extractor-type fan, the tube sits in smoothly flowing air,
mostly takes the easy way through the wide open spaces, just as Steve
says. A thin surface layer of hot air tends to stick to the glass
surface of the tube, acting as an insulating blanket that keeps the
in. Meanwhile, the valuable fan turbulence is being completely wasted
into the room. In every sense, this arrangement sucks.
If the fan is upstream of the tube and blowing air onto it, the
turbulence scrubs away that insulating blanket. The glass is in direct
contact with cool air, and the surface temperature is much lower.
If users want a quiet amplifier above everything else, that fan
turbulence is a hugely important asset - absolutely not to be
But the tube isn't the only thing that needs to be cooled. Most modern
desktop amplifiers use 'full-flow ducted cooling', which draws cool
inward past the transformer, electrolytics and other parts of the
supply, before blasting the air directly at the tube(s). This tends to
place the fan somewhere inside the middle of the cabinet, which helps
reduce noise. Finally, the hot air should be ducted directly out of
cabinet - not blasted at the tank circuit.
It isn't easy to get all of these things right. Every plan has problems as
we're looking for the best possible compromises.
73 from Ian GM3SEK>
We use a Drake L4B amp in our Multi CW contesting station. When first used,
cabinet top became very hot during 200+QSO/hr runs. Two muffin extractor fans
placed over each 3-500Z reduced the cabinet temperature to barely warm.
Maybe the Drake blower is too small for the job or is becoming tired? (a bit
original power supply, since replaced for both reasons). This ploy does not
'negative pressure' for the blower - it merely pulls in more cool air through
and top of the cabinet.
This practice is not recommended unless you are prepared to clean out the dust
and dead moths etc. between contests.
Another amp here has a filter in front of the blower intake - I change the
each contest, and the inside of the amplifier is pristine.
As Ian says, it's a compromise. <G>
73, Ken ZL1AIH (Team ZM1A in CW contests)
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