[Amps] muffin fans

[Ian White, G3SEK]

Partain, Chuck wrote:
>
>anyone ever do any studies about muffin fans for cooling BIG tubes?
>I have a decent size blower I'm going to use on my amp (gu-84b) and wondered
>if there was a smaller fan (I'm into the size reduction mode now) to use.
>they indicate ~60cfm at 30mm H2O across the tube. I have a squirrel cage
>that's going to work but would pay for a quieter smaller hi performance fan
>to do the trick. Looked around and see a lot of CFM measurements but not
>including the pressure.

That's because muffin-type fans have very poor performance against the 
kind of back-pressure created by the anode cooler of a ceramic tube. 
They are good for delivering high volume of air when the back-pressure 
is low, but any small back-pressure will decrease the CFM by a *lot*.

The rated CFM figure is always against zero back-pressure ("free 
discharge"). In other words, it's a theoretical maximum value that 
you'll never see in practice.

Squirrel-cage blowers have several features designed to develop pressure 
as well as deliver volume, so that's what you're going to need.


-- 
73 from Ian G3SEK         'In Practice' columnist for RadCom (RSGB)

http://www.ifwtech.co.uk/g3sek