Re: [Amps] Cooling requirements GS35B

[Tony King - W4ZT <amps@w4zt.com>]

Hi Robert,

The air flow of 90 cfm is published but I haven't seen any reference to 
back pressure in any documents anywhere. Back pressure depends on the 
capacity of your blower and if you achieve the 90 cfm, the back pressure 
shouldn't matter except maybe to your blower. More air is better to the 
point it blows the tube out!  It's been suggested before to use a large 
plastic garbage bag drawn tightly over the top of the tube to do a rough 
estimate of the airflow from  your blower through your tube. Roughly 
estimate the volume of the bag and then measure the time to fill it. If 
you can measure (roughly) 90 cfm or more through the tube, then you're 
getting the rated cooling. You can measure back pressure with a plastic 
tube and a column of water if it is important to you.

I'd like to add that the BOTTOM of the GS-35B including the grid ring 
will get VERY hot with nothing but filament voltage on the tube so it is 
very important to keep air flowing even with no anode voltage applied. 
In fact, it will get too hot to hold in your hand if you don't put 
significant air around the base. Be sure that your grid ring is firmly 
clamped to a metal plate (thicker is better) to help dissipate the heat. 
You've got about 38 watts from the filament plus the grid dissipation to 
deal with.

I've got a collection of GS-35B info on my web page: <http://amp.w4zt.com>

73, Tony W4ZT

Robert Thain wrote:
> Hi 
> I'm looking for the cooling requirements for a GS35B.
> I understand that flow rate AND back pressure are
> important and usually quoted. But I just cant find
> this info.
> If I understand correctly:
> Flow rate equates to the volume of air needed
> and back pressure equates to the force of the air
> needed.
> Thanks
> Robert
> G0HGW@yahoo.com

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