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Re: [Amps] Alpha 77 (8877) exhaust temp?

To: AMPS <amps@contesting.com>
Subject: Re: [Amps] Alpha 77 (8877) exhaust temp?
From: "Bill, W6WRT" <dezrat1242@yahoo.com>
Reply-to: dezrat1242@yahoo.com
Date: Mon, 06 Jul 2009 09:56:11 -0700
List-post: <amps@contesting.com">mailto:amps@contesting.com>
ORIGINAL MESSAGE:

On Sun, 05 Jul 2009 15:28:27 -0700, Jim Barber <audioguy@charter.net> wrote:

>All this talk about seals, temperatures and sensors has me wondering 
>what would be a reasonable, conservative guessestimate of the maximum 
>safe exhaust temperature on an Alpha 77? (single 8877, grounded grid)
>
>I really need to rebuild the blower anyway; a potential project might be 
>a temp sensor peeking into the silicone-rubber chimney and a small board 
>with a uP and husky PWM fan speed controller. It would require 
>converting the blower over to a (potentially larger) DC unit, but I've 
>seen that done before. It could stand the addition of some rubber 
>isolation gaskets while we're at it, too. (transmits a lot of vibration 
>through my station desk)
>
>Any thoughts?

REPLY:

I don't have data on the 77, but two other Alphas I have owned, a 91b and an 89,
both had exhaust temps that stabilized around 190 F after prolonged brick on the
key TX. I suspect the 77 would be in the same ballpark. 

My current homebrew 8877 amp with a 91 CFM blower stabilizes around 140 F under
the same conditions. 91 CFM is overkill for a single 8877 but it makes me feel
good. :-)

One thought on your proposed blower change: If you get a blower which uses a
"run" capacitor, you can switch in different values to change speed. For
example, my EBM-Pabst blower requires a 4 mF cap for full speed. If I had a one
or two mF it would run slower, and I could switch in the extra capacitance when
needed. Just an idea.

73, Bill W6WRT
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