[Amps] Advice

[Paul Decker]

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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