[Amps] 8877 blower requirement ?

[Mike McCarthy, W1NR]

Bob Alexander wrote:
> Hugh,
> The 4C440 only has 60 CFM at 0 static pressure... an 8877 needs around 40
> cfm at .6" SP. and the 4c440 does not meet this spec.
> I do not know the specs on the 2C610  but it is probably around 70CFM at .6
> SP so it would be
> the best choice of the two....even though noisier.
> 73, Bob W5AH
> 
> -----Original Message-----
> From: amps-bounces at contesting.com [mailto:amps-bounces at contesting.com]On
> Behalf Of Hugh Duff
> Sent: Thursday, April 05, 2007 6:51 AM
> To: amps at contesting.com
> Subject: [Amps] 8877 blower requirement ?
> 
> 
> I have just about finished building a W6PO style 8877 amplifier for 2
> metres and I now need to install a blower. I have a 2C610 (140 CFM)  and
> a 4C440 (60 CFM) and I'm wondering which one, if either, would be
> suitable. The latter would be desireable for lower noise but I want to
> make sure the tube is sufficiently cooled without overdoing it.  The
> Eimac datasheet that I have simply states "Cooling: Forced Air" with no
> CFM airflow specified.
> 
> I've seen rogue statements to the effect of "the bigger (more CFM) the
> better" but according to the Econco "Tube Maintenance" document, too
> much cooling may affect the electron emission of the filament.
> 
> So I'm curious to know what the optimum CFM requirement is to
> sufficiently cool a single 3CX1500a7.
> 
> 
> 73 de Hugh VA3TO
> www.va3to.com
> 
> 
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> 

The 3CX1500A7/8877 lists the minimum airflow and static pressure 
required for adequate cooling at various power levels and altitudes. 
For example at the maximum 1500W dissipation level 38 CFM at .60 inches 
of pressure is required.  At 10,000 ft. the requirements are 55.5 CFM at 
.88 inches of pressure.  However, an 8877 at 1000W dissipation (more in 
line with Amateur legal limits) needs 20.3 CFM at .23 inches of pressure 
when operated at sea level.

The manufacturer data sheet for the blower should have the CFM/pressure 
curves to determine the correct requirements.  A 4C440 blower drops to 
23 CFM at .5 inches pressure, so it is not adequate to cool an 8877 at 
full 1500W dissipation at sea level. But, the 4C440 will deliver 49 CFM 
at .3 inches, so it appears to be more than adequate for 1500W OUTPUT 
applications.

See the full data sheet at http://www.g8wrb.org/triodes.php

The blower specs can be found on http://www.grainger.com

Mike, W1NR