> When I read this I conclude that my exciter (TS-870S)
> should be run to full
> 100w output to the amp to avoid "ALC overshoot" and
> tuning/loading problems.
> Based on the responses so far, I can either 1) reduce my
> exciter drive
> level, or 2) run the full 100w output but swamp the amp
> input down to the
> needed input drive level.
ALC response in most rigs is terrible. It especially gets
worse when output power is reduced.
My FT1000D doesn't have that problem at all (since it has a
real drive control) , but my ICOM 706 and 775DSP have
terrible leading edge overshoot.
My FT1000MP MKV had that problem, but I went into a hidden
menu and turn the transmitter IF gain down and nearly
eliminated the problem at 100-200 watts. If I reduce the
output power to 50 watts, it all comes back unless I reduce
the transmitter's IF gain further.
If you have one of the exceptional rigs that does NOT have
this problem, you can safely reduce drive and load the PA at
reduced drive. Of course you must be sure you never exceed
that drive value on peaks.
Rigs have this problem because the ALC loop used for power
control has a finite response time. If the envelope tries to
rise faster than the ALC is responding a sharp leading edge
peak develops. My 775DSP for example produced over 250 watts
of leading edge peak power spike whether the power output
was set at 50 watts or 150 watts.
A second issue is IM from the transmitter and other spurious
signals. Most transmitters have a sweet spot a bit less than
full output. When peak power is above or below that level IM
performance can suffer. You want the radio to spend as much
time as possible around that power value. When you reduce
drive without reducing transmitter IF gain things like
carrier suppression and distortion can suffer.
The safest thing to do is to load the PA for the maximum
possible drive the exciter will put out and then reduce
power level at the exciter. You can get into a problem of
course if the maximum drive power exceeds the safe input
power of the amp, but this would be a very bad combination
of exciter and amplifier. I would never use an amplifier
that drove with 30 watts to absolute maximum power and
attempt to use it with a 100 watt radio. I'd never attempt
to use my 200 watt rigs with 50 watt drive amps, it's just
asking for trouble.
73 Tom
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