The problem is back pressure on the fan. Most axial fans do not do
well with much back pressure. The flow drops sharply. Squirrel cage
type blowers are usually needed for those type tubes. For a 4cx1000
type tube you need a minimum of a 3 inch wheel on the blower at 3100
rpm and that is with a wide open socket and air path. A smaller blower
would need to be higher speed.

73
Gary K4FMX

On Mon, 18 Apr 2005 19:38:11 +0200, Patrick wrote:

Hi Skipp,

Thanks for the advice.

Yes, in fact must the base be cooled as well !
I don't think the air that will go to the base will be hot, the air will
be under pressure in the anode cabinet and the hotest air will be
created on the anode cooler and be exhausted almost immediately.

I have seen a description where there are 2 Papst 3472 fans, one blowing
and one extracting the air. These fans can produce abt 36 CFM at a
very low noise level. Maybe a not too bad idea compared to a squirel
radial fan that produces the same volume but at a much higher noise level !!

Any suggestions ??

73 Patrick

Skipp a écrit :
You need to have cool air moving across the tube base pins. The
conventional method of under-up cooling supplies the base pins
with safe cooling. If you blow into the anode, what happens to
the air reaching the base pins/contacts? The seal around the
tube base contacts also needs cooling and it should not be already
hot air from passing by the anode.

Smaller tubes are forgiving in cost and operation. Jump to
the 4cx1000/4cx1500 size and you had best not over heat the
tube, both base pins and anode.

cheers,
skipp

www.radiowrench.com/sonic I have a diagram of the Collins 30S-1 available
(as well as many others).



: Patrick wrote:
: Hi All,

: I'm about to build a 4CX1500B amplifier for 2m.

: Most, if not all, descriptions are blowing air into the enclosure on the
: base side of the tube and the air goes through the socket and finaly
: through the anode chimney.
: For several reasons i would like to blow into the anode enclosure
: directly onto the anode, the air would then go out through the anode
: chimney. Only a small part of the air would cool the base of the tube
: through small holes in the anode/cathode separation.

: Is that a stupid idea ? Is there any reason not doing so ?

: 73, Patrick