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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