Hi all,
As Ian said, the cooler is attached using a a slotted nut.
I remember that someone had the idea of using heatsink
compound between the anode and its cooler for improving
the tube cooling.
I also remember that for a GI7 which I have it home,
I removed the anode cooler and saw indeed that the cooler
didn't make properly contact with the anode (but only for a
small area near the anode screw).
The best is using a blower, but where a muffin fan is the
only way, the GI7 is more suitable because of lower pressure
drop (the anode air gaps are bigger than for other tubes) and
the possibility of direct connection of the large grid ring
to the chassis (and this way some conduction cooling of
the tube body).
Most of the home made linears that I saw here and in the
ex USSR are using 5" fans, even for some larger tubes
like the GU74, GU43B and even for the 2.5kW GU78B!
(it is hard to find here good blowers for the amps,
not so easy as with some tubes, hi!).
For the 1296 MHz, the GI7 may have problems regarding
the amplifier detuning because of grid heating
(higher dissipation because of high freq.) which tend
to some changes for the tube capacitance.
Water cooling keep the tube temperature
lower and may reduce this effect.
73's,
Traian
"Ian White, G3SEK" wrote:
>
> One big slotted nut, leaving a round flat surface and a threaded post.
> The original anode cooler is sand-cast, with a relatively small number
> of fins, so it really isn't very effective and transferring the heat out
> to the air-stream.
>
> Once upon I time, I thought of replacing it with a much more efficient
> anode cooler from a dead Eimac tube (4CX250 or larger) and making an
> adaptor piece from solid copper... but I never got around to it.
>
> However, I doubt that a muffin fan would be suitable for cooling any
> "external anode" tube unless you use *really* big fins with a minimal
> pressure drop. Muffin fans are very good for moving large volumes of air
> where there is no flow restriction, but they are not good at forcing air
> through the small gaps in a typical finned anode cooler. For that you
> need a centrifugal (squirrel-cage) blower.
>
> People who use the GI-7b for 1296MHz generally change to water-cooling,
> using a water jacket that slips over the threaded post and makes good
> contact with the flat surface.
>
> --
> 73 from Ian G3SEK Editor, 'The VHF/UHF DX Book'
> 'In Practice' columnist for RadCom (RSGB)
> http://www.ifwtech.com/g3sek
>
> --
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