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Re: [Amps] Low Power

To: <d.cutter@ntlworld.com>, "Roger" <sub1@rogerhalstead.com>
Subject: Re: [Amps] Low Power
From: "Carl" <km1h@jeremy.mv.com>
Date: Mon, 2 Mar 2009 12:01:52 -0500
List-post: <amps@contesting.com">mailto:amps@contesting.com>
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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