Kevin,
Have you or anyone else figured out where to cut a Micor PA board
and modify for a 50 ohm output what is remaining to be able to use
the same board but at a lower power?
Ie: take a 100 watt PA Board, cut off the 4 transistor finals and
use the driver stage?
Neil - WA6KLA
Kevin Custer wrote:
>
> In Class C operation, especially when considering repeater operation,
> its way better to run an appropriate sized amplifier full bore than it
> is to run a bigger amp at 1/2 or 1/3 its rating. Class C amps run
> cleaner when properly (over) driven, as they will get spurious when
> under driven. Typically, the smaller amplifier will run more
> efficiently (more power for less or same amount of heat) then its big
> brother throttled back. The efficiency rating of a power amplifier will
> vary to some degree in relation to how hard the amplifier is driven.
> Usually the amp will be more efficient when properly driven.
>
> The text below is a generalization of what can happen when commercial
> radios are tuned to the ham bands and are not being driven correctly.
> My example below is from the Motorola line, but, this can happen with
> GE, Motorola, or whatever.
>
> Poor efficiency is what kills power amplifiers. Too many folks believe
> its better to run an oversized amp at 1/2 its rating to make it last.
> Lets take the Motorola Micor 100 watt UHF mobile for example. The
> amplifier in this mobile throttled back to say 50 watts of output will
> require about 12 to 15 amps of DC current. A 45 watt model running full
> bore (45 to 50 watts out) will require 6 to 8 amps of current. The 45
> watt mobile radio will produce about 45 to 60 watts of wasted heat that
> is dissipated by the heatsink. The Micor mobile heatsink is rated at
> around 40 watts continuous dissipation, so we are close to being able to
> run it continuous duty, and can likely stretch that extra 5 to 20 watts
> of dissipation if a fan is used. Now lets take the 100 watt mobile that
> is throttled back to 50 watts. Its dissipating 117 watts of heat, and
> won't matter how many fans you blow on it, it's going to eventually burn up.
>
> What I'm driving at here is *Dissipation* and *Efficiency* are words
> that we as repeater builders need to understand.
>
> In my opinion, its better to appropriately size the power amplifier and
> save the additional money on the electric bill then to be replacing the
> PA every 6 months due to heat failure.
>
> Kevin Custer
>
> georgiaskywarn wrote:
>
> >Good Morning Tom,
> >I would have to run the driver just about on full bore though.
> >(because of the 30 in 100 out amp...again only running it right now at
> >75 watts) I am a little nervous on running it like that. I wonder if
> >anyone else is running the driver that hard and getting good results??
> >Thanks,
> >Robert
> >
>
>
> Yahoo! Groups Links
>
>
>
>
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