I sometimes wonder if the published specs are on the conservative side.
Take the Amp Supply LK-800 series for example. It runs 3 3CX800's. The
cooling is by a Papst muffin fan mounted in the small enclosed (pressurized)
tube chassis. Of all the 800's Ive worked on for service or 6M conversion
Ive yet to see a heat blackened tube and this includes the -C and TNY
versions which can get close to 5KW out with 3CPX tubes.
The same lack of heat damage hold for the older Titan which uses a real
blower.
Look at any of the 8874 Alphas. After decades of use when the tubes are
barely showing gain they look almost new.
OTOH hand industrial/medical pulls are often heat discolored.
Carl
KM1H
----- Original Message -----
From: "Roger" <sub1@rogerhalstead.com>
Cc: <amps@contesting.com>
Sent: Sunday, December 06, 2009 2:09 AM
Subject: Re: [Amps] Advice
> Cooling flow and pressure are like voltage, amps, and resistance
> the figures as given in the data sheets tell two things. That the tube
> needs a given flow in CFM for a given power and that the "fan" must be
> capable of delivering so much pressure at the socket.
>
> With that given, if the tube (3CX-800) requires 19 CFM you *WILL* have
> 0.50" of pressure at the socket. Conversely IF you have 0.50 inches at
> the socket you will have 19 CFM. However if there is a down stream
> restriction the pressure will rise and the flow will drop which would be
> quite unusual. I'd not think this would be a normal situation, but when
> designing an amp it is possible to not leave enough venting area for the
> cooling to maintain proper flow.
>
> Possibly one of the reasons for the difference between the data sheets
> would be a redesign of the anode cooler fins. Another of course could be
> a change to a more conservative CFM per watt
>
> One point is the lower the pressure and/or flow the less efficient the
> heat transfer and it is not linear as far as I can see.
>
> 73
>
> Roger (K8RI)
>
> Joe Subich, W4TV wrote:
>> The current Eimac data sheet gives the cooling requirements
>> at sea level as:
>>
>> Anode CFM Pressure
>> Diss In. Water
>> ---------------------------------
>> 400 6 0.09
>> 600 11 0.20
>> 800 19 0.50
>>
>>
>> However, the original (1983) Eimac data sheet for the 3CX800
>> gave the following:
>>
>> Anode CFM Pressure
>> Diss In. Water
>> ---------------------------------
>> 800 19 0.35
>>
>>
>>
>>> The cooling requirements are the same per watt of dissipation
>>> for the 8877 as they are for the 3CX800.
>>>
>>
>> Not according to the Eimac data sheet (1971) for the 8877
>>
>> Anode CFM Pressure
>> Diss In. Water
>> ---------------------------------
>> 500 7.7 0.10
>> 1000 20.3 0.23
>>
>>
>> Each of the points has a different cfm/W value:
>> 6/400 <> 19/800 <> 7.7/500 <> 20.3/1000
>>
>> 73,
>>
>> ... Joe, W4TV
>>
>>
>>
>>> -----Original Message-----
>>> From: amps-bounces@contesting.com
>>> [mailto:amps-bounces@contesting.com] On Behalf Of Paul Decker
>>> Sent: Sunday, December 06, 2009 12:11 AM
>>> To: garyschafer@comcast.net
>>> Cc: amps@contesting.com
>>> Subject: Re: [Amps] Advice
>>>
>>>
>>>
>>>
>>> Hi Gary,
>>>
>>>
>>>
>>> I don't know if it's a linear relation. Perhaps a typo, but
>>> on page 13.19 of the 1995 handbook, it says 12 CFM at 0.09"
>>> back pressure for two 3cx800's operating at 400 Watts
>>> dissipation each.
>>>
>>>
>>>
>>> Paul (KG7HF)
>>>
>>>
>>> ----- Original Message -----
>>> From: "Gary Schafer" <garyschafer@comcast.net>
>>> To: "Paul Decker" <kg7hf@comcast.net>
>>> Cc: amps@contesting.com
>>> Sent: Saturday, December 5, 2009 11:57:34 PM GMT -05:00
>>> US/Canada Eastern
>>> Subject: RE: [Amps] Advice
>>>
>>>
>>>
>>>
>>> Hi Paul,
>>>
>>> I didn’t mean that I didn’t understand the 2 nd paragraph but
>>> that I didn’t read it. I just overlooked it when I read your
>>> first post with it.
>>>
>>>
>>>
>>> I do think that you have the cooling requirements wrong for
>>> the 3CX800 though. I got the info from the 2000 ARRL handbook
>>> section on amplifiers. There is a table there that list the
>>> cooling requirements.
>>>
>>> The cooling requirements are the same per watt of dissipation
>>> for the 8877 as they are for the 3CX800.
>>>
>>>
>>>
>>> By the way I see that I made a type in one place where I show
>>> .03333 cu ft/watt. It should be .02333 cu ft/watt.
>>>
>>>
>>>
>>> At 400 watts dissipation each they should require around
>>> (.02333 x 400) 9.3 cu ft of air each or 18.6 for a pair for
>>> 800 watts dissipation.
>>>
>>>
>>>
>>> A single 3CX800 operated at 800 watts dissipation would
>>> require 19 cu ft of air per the chart at .5 back pressure.
>>>
>>>
>>>
>>> 73
>>>
>>> Gary K4FMX
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>> From: Paul Decker [mailto:kg7hf@comcast.net]
>>> Sent: Saturday, December 05, 2009 6:26 PM
>>> To: garyschafer@comcast.net
>>> Cc: amps@contesting.com
>>> Subject: Re: [Amps] Advice
>>>
>>>
>>>
>>>
>>> Hi Gary,
>>>
>>> The point of the second paragraph was that with a pair tubes
>>> operating under their specificed dissipation rating require
>>> less cooling then a single tube operating at or above its
>>> dissipation range.
>>>
>>>
>>>
>>> Two 3cx800's operating at legal limit only require 12 CFM at
>>> about 0.09 - 0.1" of back pressure, this is because the tubes
>>> are only operating at 400W dissipation each.
>>>
>>>
>>>
>>>
>>>
>>> Paul Decker (KG7HF)
>>>
>>>
>>> ----- Original Message -----
>>> From: "Gary Schafer" <garyschafer@comcast.net>
>>> To: garyschafer@comcast.net, "Paul Decker"
>>> <kg7hf@comcast.net>, amps@contesting.com
>>> Sent: Saturday, December 5, 2009 5:11:10 PM GMT -05:00
>>> US/Canada Eastern
>>> Subject: RE: [Amps] Advice
>>>
>>> Paul, sorry I missed your second paragraph.
>>> Air flow needed depends on how much power is being
>>> dissipated. If you divide
>>> the airflow by the dissipation rating of the 8877 as an
>>> example 35/1500 =
>>> .02333 cu ft/watt. multiply that by 1600 (for a pair of
>>> 3CX800s) and you get
>>> 37.3 cu ft of air flow. The cooling efficiency of both are
>>> about the same
>>> .02333 cu ft/watt.
>>>
>>> So if you ran the pair of 3CX800s at the same power level as
>>> the 3CX1500 it
>>> would require just about the same amount of air flow. .03333
>>> x 1500 on the
>>> pair of 3CX800s = 35 cu ft.
>>>
>>> I don't know what the back pressure would look like on the
>>> pair of 3CX800s
>>> at reduced air flow but it would probably be pretty close to
>>> the 3CX1500.
>>>
>>> 73
>>> Gary K4FMX
>>>
>>>
>>>> -----Original Message-----
>>>> From: amps-bounces@contesting.com
>>>>
>>> [mailto:amps-bounces@contesting.com]
>>>
>>>> On Behalf Of Gary Schafer
>>>> Sent: Saturday, December 05, 2009 4:40 PM
>>>> To: 'Paul Decker'; amps@contesting.com
>>>> Subject: Re: [Amps] Advice
>>>>
>>>> The chart that I am looking at says: 8877 requires 35 cu ft at .41
>>>> back
>>>> pressure.
>>>>
>>>> A 3CX800A7 requires 19 cu ft at .5 back pressure. Two of those would
>>>> require
>>>> 38 cu ft at .5 back pressure.
>>>>
>>>> 73
>>>> Gary K4FMX
>>>>
>>>>
>>>>
>>>>> -----Original Message-----
>>>>> From: amps-bounces@contesting.com
>>>>>
>>> [mailto:amps-bounces@contesting.com]
>>>
>>>>> On Behalf Of Paul Decker
>>>>> Sent: Saturday, December 05, 2009 4:26 PM
>>>>> To: amps@contesting.com
>>>>> Subject: Re: [Amps] Advice
>>>>>
>>>>>
>>>>>
>>>>> When I was writing the original reply I was thinking more
>>>>>
>>> along the
>>>
>>>>> lines of different tubes for example a single 8877 requires more
>>>>>
>>>> airflow
>>>>
>>>>> and has more back pressure than two 3cx800's.
>>>>>
>>>>>
>>>>>
>>>>> Agreed, two tubes of the same type would require more
>>>>>
>>> airflow than a
>>>
>>>>> single tube. However, I think we could assume that two
>>>>>
>>> tubes of the
>>>
>>>>> same type would produce 2x the power of the single tube
>>>>>
>>> amp. If the
>>>
>>>>> twin tube amp were run at the same output as the single holer, it
>>>>>
>>>> would
>>>>
>>>>> also follow to reason the airflow requirements could for the two
>>>>> holer
>>>>> could be reduced because each tube is only being driven
>>>>>
>>> to a reduced
>>>
>>>>> output.
>>>>>
>>>>>
>>>>>
>>>>> Paul
>>>>>
>>>>>
>>>>>
>>>>>
>>>>> ----- Original Message -----
>>>>> From: "Gary Schafer" < garyschafer@comcast.net >
>>>>> To: < dezrat1242@yahoo.com >; < amps@contesting.com >
>>>>> Sent: Saturday, December 05, 2009 11:04 AM
>>>>> Subject: Re: [Amps] Advice
>>>>>
>>>>>
>>>>> There is no free lunch. While it may seem like you could
>>>>>
>>> get by with
>>>
>>>>> less
>>>>> air flow it doesn't follow.
>>>>> A pair of tubes will require twice the air flow at the same back
>>>>> pressure as
>>>>> a single tube. Tubes being the same of course.
>>>>>
>>>>> 73
>>>>> Gary K4FMX
>>>>>
>>>>>
>>>>>
>>>>> _______________________________________________
>>>>> Amps mailing list
>>>>> Amps@contesting.com
>>>>> http://lists.contesting.com/mailman/listinfo/amps
>>>>>
>>>> _______________________________________________
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>>>>
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>>>
>>>
>>
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>>
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