[Amps] Coupling a blower to an air system socket

[Jim Garland]

 Very interesting concept, Ian. I'm wondering how the 25-30% downward flow
past the filament pins is adjusted? Presumably one needs to size an outside
vent on the underchassis to exhaust that air. Also, I don't quite understand
why the back pressure on the blower is reduced. Seems like most of the air
has to flow up through the anode cooler, which presumably is the largest
flow impedance. The air flowing past the filament pins has to have a
bottleneck where it vents from the enclosure to keep the flow down to
25-30%. . The combination of the two vents (the filament vent and anode
cooler vent) presumably reduces the back pressure slightly, but I wouldn't
think the effect would be very great.

On a related topic: In my experience, a problem with blowers is often that
the motor rpm is too high, causing turbulence in the airflow. As noted by
somebodly else, turbulent flow is less effective at cooling an anode than
laminar air flow. This fact was known by the Collins engineers who designed
the 30S-1 cooling system. They mounted a low speed blower directly under the
tube socket, powered to give laminar airflow through the 4CX1000A.  Some
hams (misguidely, in my opinion) swap the orignal 4CX1000A for a 4CX1500B,
in the hope that the 1500W plate dissipation of the latter tube willl
provide a larger safety margin. Unfortunately, the reverse happens, because
the fins in the 4CX1500B are much more densly packed, which inserts
additional flow impedance into the air  path and causes turbulent flow. The
actual net effect is to reduce the cooling and, hence, decrease the
amplifier performance. 
73,
Jim W8ZR


73,
Jim W8ZR, 
> Another very effective method of cooling is to blow air directly into a
> sealed anode compartment. Most of the air flows upward through the anode
> cooler and is vented directly to the outside through a chimney ABOVE the
> anode cooler. There is NO chimney between the base and the anode cooler.
> Meanwhile 25-30% of the air flow is allowed to bleed downward through
> the tube socket to cool the base seals. This method reduces the back
> pressure on the blower, and allows it to deliver much more air than the
> conventional base-upward layout.  It has been used very successfully for
> decades in VHF and UHF amps - so much so, it is regarded as "the normal
> method".
> 
> 
> 73 from Ian GM3SEK
>