[Amps] Advice
Joe Subich, W4TV
lists at subich.com
Sat Dec 5 21:40:46 PST 2009
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 at contesting.com
> [mailto:amps-bounces at contesting.com] On Behalf Of Paul Decker
> Sent: Sunday, December 06, 2009 12:11 AM
> To: garyschafer at comcast.net
> Cc: amps at 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 at comcast.net>
> To: "Paul Decker" <kg7hf at comcast.net>
> Cc: amps at 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 at comcast.net]
> Sent: Saturday, December 05, 2009 6:26 PM
> To: garyschafer at comcast.net
> Cc: amps at 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 at comcast.net>
> To: garyschafer at comcast.net, "Paul Decker"
> <kg7hf at comcast.net>, amps at 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 at contesting.com
> [mailto:amps-bounces at contesting.com]
> > On Behalf Of Gary Schafer
> > Sent: Saturday, December 05, 2009 4:40 PM
> > To: 'Paul Decker'; amps at 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 at contesting.com
> [mailto:amps-bounces at contesting.com]
> > > On Behalf Of Paul Decker
> > > Sent: Saturday, December 05, 2009 4:26 PM
> > > To: amps at 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 at comcast.net >
> > > To: < dezrat1242 at yahoo.com >; < amps at 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
> > >
> > >
> > >
> > > _______________________________________________
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> >
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