Steve Katz wrote:
> [Steve Katz] Thanks. But what I do is use thermally sensitive
>indicating strips installed on the critical components to see if I'm
>really making any difference or not. Those are very inexpensive and
>work well. I install them next to the anode cap on the tube envelope,
>between the filament pins on the tube base, on filter capacitor cases,
>and anywhere I think heat may be a problem. Then, I run with the
>"original" cooling system, record all the measurements on paper, and
>make the modifications and run with the modified cooling system, and
>record all those measurements. It becomes pretty obvious if I'm
>cooling the "right stuff" with my boost
>cooling, or not.
> -WB2WIK/6
>
I've used the indicating strips to check the temperatures on a 3-500Z in
a SB-1000, with quite interesting results.
As most of you will know, the SB-1K has a large muffin-type fan in the
center partition wall, blowing directly onto the 3-500Z. Cool air is
sucked in through holes in the side to the cover, which kind of direct
it through the electrolytic stack and around the transformer before it
reaches the fan. Exit of hot air is through holes in the side of the
cover, on the opposite side of the tube to the fan, so the air has to
flow around the tube to get to the exit.
The tests were not very scientific because the only strips available
were 211-260C, and at $1 a time I wasn't going to burn up too many.
However, I did try to keep the DC/RF conditions constant, with the anode
at medium red heat.
Without the cover - so the air was blowing onto the upstream side of the
tube, but then going wherever it wanted - the upstream side stayed at
220-230C but the downstream side very quickly went off-scale.
With the cover, the total airflow decreased due to the additional
back-pressure (you could hear the fan rpm drop)... but the upstream
temperature stayed pretty much the same as before, and the downstream
temperature came down dramatically - it was maybe about 50C higher than
the upstream side (it only just went off-scale, but much more slowly
than before).
Evidently without the cover the air was not coming around to cool the
downstream side of the tube. With the cover, the exit holes close to the
downstream side of the tube were forcing the air to come back around.
That makes some kind of sense, but it has made me rather suspicious of
blowing air onto one side of the tube without also making sure some air
gets around the other side too.
--
73 from Ian G3SEK Editor, 'The VHF/UHF DX Book'
'In Practice' columnist for RadCom (RSGB)
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