[Elecraft] K3 and Expert 1K-FA

[Tom W8JI]

<<<What if you have one of the 200w or new 400w output rigs? 
What if you have
one of those radios that shoot out a spike of greater than 
100w at the
beginning of every transmission?

Do people blast the inputs of their SS amps with 200w or 
400w? You bet they
do. They fry the FETs immediately too. We see them on a 
fairly frequent
basis and we have gotten adept at swapping out FETs 
quickly - at significant
expense to the owner of the amp. In the vast majority of 
cases, they had ALC
disconnected and made a mistake when switching antenna 
outputs from these
high-power transceivers.

So, if you feel confident that you won't make a mistake, 
then go ahead and
take the risk of not having ALC connected and adjusted 
properly. But, keep
in mind the costly consequences that can result if you make 
a mistake. >>>

A word of additional caution....

While external ALC might force a long term power reduction, 
the external ALC system generally does not correct overshoot 
in radios. It  almost never reduces power spikes on leading 
edges.

The reason ALC does not normally affect the spikes, and why 
typical external ALC systems cannot reduce the spike, is the 
response delay in the ALC system inside the radio. There is 
group delay in filter, and there is propagation delay 
through the radio. There is also delay in the actual ALC 
system. The gain control system is up front in the radio, 
almost always ahead of filters and the delays. The ALC 
sampling is after filters, and in the case of amplifier 
derived ALC it is no better than the directional coupler 
that samples power inside the radio. It is a serious mistake 
to let the external ALC, derived after the filters and group 
delay in the radio, do primary power control.

What the ALC does buy people is gain control or power 
limiting if the knob on the radio is adjusted wrong, but it 
most certainly will not correct leading edge spikes.

It's my understanding the K3 uses a two stage system, and 
handles the sampling before passing through filters. I'm 
still trying to get my head around how it works and why I am 
uncomfortable with how the meters act, but at least it 
addresses the overshoot problem.
The amplifier ALC should be connected, if  the manufacturer 
requires it, but don't expect it to make something like an 
IC 775 not pop FET's. The correct approach would be an 
attenuator pad so the radio could run at near full power and 
not have a chance of overdriving the amp, or better yet buy 
a radio that does not spike to 250 watts at the leading 
edge.

Now there is one exception to this. If the amplifier has a 
memory on ALC, and holds the ALC at the highest value and 
reduces ALC until power comes slowly up to the correct 
value,, then there would be no overshoot. I'm not aware of 
any amplifiers that do this, and I expect customers would 
complain about the slow power response and lack of 
compression or reduced average power with a system like 
that.

Without an attenuator to "match levels", people are gambling 
with or without the ALC connected.


73 Tom