A while back on another list there was a discussion about a robust amp
can not be built in a table top size. I believe this applies to solid
state amps as well. The key to building a gutsy solid state amp is the
use of multiple devices and distributing the heat over a larger area.
All solid state broadcast transmitters are comprised of multiple
modules. Sometimes multiple fans are used to cool the modules. I am not
aware any amateur linears with a modular design but there is nothing
beyond economics to prevent one from being built.
On 7/8/12 10:18 AM, Tom W8JI wrote:
> Hi Larry,
>
> All of the links and data agree with what I found here in my measurements. I
> think the real issue is some very creative marketing is being done, and the
> factory data sheets can be a bit confusing. They certainly do not contain
> linear data.
>
> Here are the main points:
>
> 1.) The "1250 watt device" is actually about a 800 watt PEP maximum device
> in linear service. At 800 watts, it is right on the edge of what we are
> accustomed to for Ham product IMD.
>
> 2.) A major problem is heat. Because all heat is in one small footprint, it
> needs a very thick machined copper spreader or liquid cooling, even at just
> 800 watts PEP.
>
> 3.) Like any other device, they will fail with mismatch at high power. They
> absolutely will require SWR shutdown and temperature monitoring.
>
> If we read all of the links carefully enough to cut through the marketing
> fluff, we will see every reference link listed (where they have actually
> tested) agrees with all of this.
>
> I think what has really happened, is many home or first-time builders have
> taken the data sheet at face value. They have convinced themselves this is a
> 1250 watt output device that can be run right into almost any load without
> worry. Of course, neither of those things are remotely true.
>
> I understand it is tough to let go of the magic of a single 1250-watt device
> that slapped on a heatsink without protection, and can be run into a 65:1
> SWR without failing. But this is really a 800 watt PEP linear device that
> comes with all the long term baggage of any other similar device, as ALL of
> those links also seem to agree with.
>
> At 600 watts it will be pretty clean. At 800 watts about at the lower limit
> of what cheap tube amps can do. All of this requiring getting the heat out,
> and shutting it off if SWR goes high.
>
> 73 Tom
>
> <<<<<Like Tom, I am convinced that it is not possible to run that part at
> full power in normal amateur service due to the heat issues. Maybe some
> energetic person will build one up for 1.8 MHz using cryogenic cooling and
> let us know how that works out...
>
> 73,
>
> Larry - W7IUV>>>>
>
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