[Amps] Fans for 4cx1000/1500 question

[Gary Schafer]

"Ian White, G3SEK" wrote:

> Richard wrote:
> >>I need to know how to determine if the following fans will be adequate
> >>for an 4cx1000/1500 amp. While I can find the cfm specs, I cannot
> >>find the back pressure specs.
> >
> >Ordinary fans do not create enough air pressure to cool external anode
> >tubes. A centrifugal blower makes more pressure.  I like to mount the
> >tube/socket in a cardboard box and measure air pressure with a
> >manometer.
> >
> >
> Agreed about the cardboard box mock-up, just to see if the blower is big
> enough.
>
> Broadening the answer, it is better to measure the cfm directly by
> letting the exhaust air blow up a thin plastic bag, and timing how long
> it takes (as recommended here by K6GT). I use a 55 litre swing-bin bag
> which typically inflates from flat to full in a few seconds... don't
> know what size they are in the USA.
>
> Measuring cfm also allows you to use other cooling methods, which may be
> better than the manufacturer's standard system of blowing in through the
> input compartment, through the socket and out through the anode (aka the
> missionary system).
>
> Blowing into a sealed output compartment and letting the hot air out
> through a chimney on the *top* of the tube has many advantages.
> Typically 25-30% of the airflow is bled down through the socket and out
> through an exhaust port in the input compartment.
>
> This method has been very widely used for the last 30-40 years in VHF
> amps, where the necessary sealed output compartment is standard
> practice. Advantages are:
>
> 1. It circulates *cold* air all around the tank components, unlike the
> normal system which circulates hot air.
>
> 2. The pressure drops upwards through the anode cooler and downwards
> through the socket are now in parallel, not in series as in the normal
> method, so the blower has to provide a lower pressure. That generally
> means that the same blower will provide *much* more airflow through the
> anode in this configuration, as well as cooling the base of the tube
> very adequately.
>
> An hour or so spent with cardboard boxes, duct tape and a plastic bag
> will show very clearly which method is superior, and whether your blower
> is big enough.
>
> I wouldn't recommend every sucking hot air into the amplifier, or
> through the blower.
>
> --
> 73 from Ian G3SEK          Editor, 'The VHF/UHF DX Book'
>                            'In Practice' columnist for RadCom (RSGB)
>                             http://www.ifwtech.com/g3sek
>

Interesting thoughts Ian. Would you think that pressurizing the output
compartment on an hf amplifier with a tube like a 4CX250 or a 4CX1000 would
be practical?
It sure would help on the 4CX250 if some back pressure relief could be had.
Looking at that tube though it seems that most of the air would go out the
bottom rather than the dense plate heat sink.

73
Gary  K4FMX