[Amps] drive power

[Tom W8JI]

> 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