Why not accept a small signal loss and run at 60-70W with a small fan if
necessary? That can be recovered with a slightly better antenna.
Carl
KM1H
----- Original Message -----
From: <d.cutter@ntlworld.com>
To: "Roger" <sub1@rogerhalstead.com>
Cc: <Amps@contesting.com>
Sent: Monday, March 02, 2009 5:44 AM
Subject: Re: [Amps] Low Power
> The other aspect of running constantly at 100W is the temperature of
> the finals and drivers: that would concern me much more. The MTBF
> drops like a stone at constant high temperature; everything else in
> the box will also be at raised temperature. I don't know how good the
> Pro is at shifting heat, but it's a general principle when calculating
> MTBF to MIL STD 217. Your linear is built to cope with much higher
> than 100W average power without much affecting its reliability
> ratings.
>
> David
> G3UNA
>
>
> ---- Roger <sub1@rogerhalstead.com> wrote:
>>
>>
>> Mike wrote:
>> > I seem to remember reading someplace that the final Transistors
>> > used in most
>> > 100 watt rigs are do not give a very clean output at 10 watts or
>> > so, 25
>> > watts and up the output is clean, now this normally would not make
>> > any
>> > difference if you were just running 10 watts QRP because no one can
>> > hear
>> > you. But, when you amplify that 10 watts with an external
>> > amplifier then
>> > who knows how it may spatter across the band. Anyway just a
>> > thought, if you
>> > have a Scope it might be worth taking a look at the signal.
>> >
>> >
>> It's been quite a while since I was involved at that level, but I
>> think
>> that would be dependent on the particular transistor(s) and the
>> circuit.
>> The curves for the 2SC5125 transistor listed as the two finals in the
>> Icom 756 Pro III look to be fairly linear even in the low end.
>> However
>> the impedance running 10 watts versus 100 watts out it's designed for
>> should change considerably with a fixed network. I'd think that
>> would
>> create quite a mismatch. Those are rather impressive transistors
>> rated
>> at 80 watts out each into an SWR as high as 8:1 at 175 MHz. So
>> running
>> the Pro III at the full 100 watts gives a large safety factor. That
>> being the case, with the Pro series I'd not give a second though to
>> running the thing at full power.
>>
>> OTOH I'm surprised to see transistors rated for such a high frequency
>> used as low as 1.8 MHz. In "the old days" VHF transistors didn't do
>> well at the low end of the HF spectrum. Of course back then we also
>> put
>> a string of resistors and caps across a string of diodes, something
>> no
>> longer needed.
>>
>> 73
>>
>> Roger (K8RI)
>> > Mike
>> >
>> > -----Original Message-----
>> > From: amps-bounces@contesting.com
>> > [mailto:amps-bounces@contesting.com] On
>> > Behalf Of Ken Bills
>> >
>> >
>> > While RTTY contesting this weekend, I wondered, since my amp still
>> > was
>> > warmed up, if it made sense to run the Alpha at 100 watts output,
>> > versus
>> > pushing the exciter at it's 100 watt limit. It only required about
>> > 9-10
>> > watts output, to deliver the 100 watt contest limit.
>> >
>> >
>> >
>> > I am sure this doesn't help my carbon footprint, but it may save
>> > the finals
>> > in the Icom ProIII.
>> >
>> >
>> >
>> > Any thoughts?
>> >
>> >
>> >
>> >
>> >
>> >
>> >
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