Assuming I now understand the problem of using ALC and too much drive power
for a given amp, how about just not using ALC in the same situation? An
example: use the Icom 775DSP (I have one also) with an Alpha 87A (also have
one of these and it recommends NO ALC connection).
73, Keith NM5G
----------------------------------------------------
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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