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[AMPS] Blowers

To: <>
Subject: [AMPS] Blowers
From: (Rich Measures)
Date: Fri, 13 Mar 98 04:49:01 -0800
>I said Earlier:
>> >While its probably true that if the pressure is okay, then the flow
>> is
>> >too *if you use the correct bases*, you can not apply this if you
>> make
>> >your own bases, which many amateurs do.  Neither can it be applied in
>> >the case of some of the triodes (3CX5000A7, 3CPX5000A7, YC156,
>> >3CX15000B7) and others, where you need to cut some slots in the
>> chassis
>> >for air flow.
>R. L. Measures replied.
>> It can if one measures the differential pressure drop across the anode
>> cooler with a manometer.
>> cheers
>> Rich...
>> R. L. Measures, 805-386-3734, AG6K
>According to my reasoning, this is simply not true. The Eimac data given
>is for the tube and socket used together. 

Granted, however, you were talking about a situation where an Eimac 
Socket is not used.  In such a case, I say that if one measures the 
pressure differential across the anode cooler one could determine if 
there was adequate air flow.  
>The pressure drop quoted in
>the data sheet would be the sum of the pressure drops across the socket
>and anode cooler - pressure drops add algebraically. You should not them
>compare that to the pressure drop across only the anode cooler, which
>will always be less (for the same flow) than the pressure drop quoted in
>the data sheet.
In the most common socket used for 'tetrodes with handles', the SK-300A, 
there is exceedingly little pressure drop before cooling air enters the 
anode cooler.  
>Aiming to get the same pressure drop across only the anode cooler, to
>what Eimac get across the socket/tube combination, would be a safe
>practice, but it might mean you use a bigger blower than necessary.  
>If you make an assumption, for the sake of argument, that a tube has 5"
>droped across the anode cooler, and 1" across the socket. 
>The data sheet
>would just say 6" (5+1=6). Now if you stick a manometer across only the
>anode cooler, and adjust a fan speed for  6" of pressure, you would
>infact be running at about (6/5)^2 = 1.2^2 = 1.44 x as much flow as
I do not fully agree with this assumption.  Volume lags far behind 
pressure.  Flowing double the volume requires very roughly triple the 
pressure.  For instance, the 8171 requires 0.36" of pressure drop to flow 
114 cubic feet of air per minute at sea level.  To flow 225 cfm, 1.0" of 
pressure drop is needed.  To flow 450 cfm, c. 3.3" is needed.  
>I dont suggest these figures are representitive, but I think
>they prove your theory does not hold.
>Perhaps I'm wrong.  Perhaps I have misunderstood your argument .....
Perhaps.  .  .  In ssb service for hams, an external anode tube little 
air pressure drop since the duty cycle is only about 15%, unless berserko 
speech processing is used.  However, for RTTY broadcasting service at the 
same PEP level, roughly eight times the pressure drop is needed.   
-  If I were going to design and build another amplifier, I would use a 
brushless DC motor turning a Rotron Centrimax (turbocompressor-type) 
centrifugical blower.  I would utilize a thermistor in the exhaust air 
stream to control the DC potential fed to the motor.  As the tube works 
harder, the blower works harder.
-   Continuing with the editorial, I would not choose to use another tube 
with indentations in the cooling fins.  (8169, 8281, et cetera which make 
a semi-stentorian noise like a gale blowing through a pine trees)  The 
venerable old 8171 has good, old fashioned flat cooling fins which make 
no such noise.  The 8171 cools quite nicely in ssb service with only 
0.45" of pressure drop.  In standby, the 8171 needs only about 0.2" of 
pressure drop.   And, as an added bonus, it has comfy handles.   'Tis a 
lovely tube indeed.  


R. L. Measures, 805-386-3734, AG6K   

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