David and Wendy Dodds wrote:
>
> Hi all.
>
> I'm almost finished putting together a 23cm cavity amp - just a single 7289
> round cavity, similar to N6CA's design.
>
> My concern is cooling. I'm using forced-air cooling, which seems pretty
> effective in that the exhaust air never gets dramatically hot, even when
> running cw for a protracted period of time. What does get hot is the anode
> tuning control, presumably due to convection along the shaft (which is only
> 1.5 inches long) The Eimac spec sheet for the 7289 shows a safe maximum
> operating temp of 250 degrees celsius, so I guess it's ok for the cavity to
> be pretty hot!
This does not answer your question (others have done that) but may be of
interest. When I was a kid, my Dad took me to see a high power RF
heating unit at his work. They had a cheap, simple ... over-temperature
sensor. Basically a "plastic band" was put around the anode and a
microswitch. If the anode got too hot, the band melts, the switch opens
... I don't know the material used, but with a temperature controlled
soldering iron, something suitable may be found. That was water cooled
and so the temperature was lower than the air cooled tubes, so it might
not be too practical at 250 deg C. I don't know the effect of the RF on
the 'plastic' too. But it is food for thought.
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Dr. David Kirkby PhD,
Senior Research Fellow,
Department of Medical Physics,
University College London,
11-20 Capper St, London, WC1E 6JA.
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