[Amps] Alpha 78 rebuild, some questions.

[TexasRF@aol.com]

Or, one can tape a big plastic garbage bag to the top of the amplifier and  
time how long it takes to fill the bag with air. If a 20 cubic foot bag  fills  
in 20 seconds then you have a flow of 60 cubic feet per minute.  Should agree 
with the temperature calculations.
 
I think credit goes to G3SEK for this technique.
 
73/k5gw
 
In a message dated 7/23/2005 8:31:32 A.M. Central Standard Time,  
david.kirkby at onetel.net writes:

Martin  Sole wrote:
> I didn't check with the internal fan only but with both  fans running and
> using my cheap Chinese multimeter with the supplied  temperature probe taped
> to the top of the case in the airstream I get  the following. Normal room
> ambient varies around 27 C depending on how  much airconditioning is 
running.
> Amp on and in standby, after the 1  minute warm up about 30 C. Normal speech
> at about 1200 W out it goes  to around 78 C and with the processor on I have
> seen about 95 C.  Second question, can I relate these figures into something
> useful to  know if or not the tubes have enough air through them?
> 
>  Thanks
> 
> Martin HS0ZED

With no RF drive applied, but in  transmit mode, the dissipation on the 
tube should be easy to calculate -  it's the total of anode, and filament 
powers.

>From the  dissipation and temperature rise, you can work out what the 
air flow is. I  can't recall the formula, but you will find in in one of 
the application  notes at http://www.g8wrb.org/.

Then look on the data sheet and see if  it is sufficient.

If you are at high altitude, you need to be a bit  more careful, as the 
formula for computing airflow from temperature rise  will need to take 
into account the lower density of the air.

This  is all based around the fact the mass of one cubic foot of air is 
known,  and the specific heat capacity of air is known. From those, you 
can work  out everything, but the formula is *really* simple. I just 
can't recall  the constant.

-- 
David Kirkby,
G8WRB

Please check out  http://www.g8wrb.org/
of if you live in Essex  http://www.southminster-branch-line.org.uk/


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