Hi Steve,
> Looking at the specs for the 2SC5125 output transistors
> used in the ICOM 756 series transceivers. Spec sheet says
> they are rated for 80W output each or 160W per pair up to
> 175Mhz.
> Wondering why ICOM only runs them at 100W?
ICOM wants to have barely tolerable IM performance on SSB,
so like most 100 watt radios they run them as high as they
dare for acceptable splatter. It would be better to run them
at 75 watts.
> There is an adjust pot that will allow the output to be
> turned up somewhat -- up to around 150W.
Unfortunately..yes. Worse yet people have found it.
> What is the undesirable effect of turning up the power to
> say... 125W, if any?
Bipolar transistors, especially low voltage transistoes,
need to be operated with a lot of headroom to have even
barely acceptable IM performance. If you look at reviews of
most radios they are in the -30 to -38dB PEP range for IM3,
and have significant stuff out to the 7th order and higher.
They are even worse on speech where the ALC and short term
voltage regulation of bias and collector voltages come into
play.
Just my opinion, but probably the worse thing we have to
look forward to is (other than CB amps making it into
amateur hands) "peak the power up" phobia. For every dB
gained on frequency several dB worth of splatter is gained.
Not a good tradeoff.
The was a FT-1000MP here for some de-click work, and I
noticed it was running 150w. It measured about -28dB PEP on
third order (ARRL method)....which is -22dB by normal
commercial standards. I backed the thing down to 100 watts
and it was around 10dB better for IM. A few months later I
saw the radio again to add some filters and it was right
back at 150 watts. Not good.
73 Tom
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