[Elecraft] K3 and Expert 1K-FA

[Bob Naumann]

Paul,

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. I do
not understand why one would not connect the ALC when its purpose is to
prevent damage. Not using ALC is not a "badge of courage"; instead, it is a
foolish tempting of fate - in my opinion.

Also, most amp manufacturers recommend that you only increase the output of
the radio high enough to achieve the desired output from the amp - they do
not recommend turning the radio output up "full-blast" and throttling the
output back down with ALC - which, as we all know can cause distortion and
all sorts of bad things. There is no downside to using ALC in the manner
that SS amp manufacturers recommend.

So, the bottom line is to follow the manufacturer's recommendation regarding
ALC and setting your radio to work properly in conjunction with it.

73,

Bob W5OV

-----Original Message-----
From: elecraft-bounces at mailman.qth.net
[mailto:elecraft-bounces at mailman.qth.net] On Behalf Of Paul Christensen
Sent: Wednesday, April 21, 2010 9:10 AM
To: elecraft at mailman.qth.net
Subject: Re: [Elecraft] K3 and Expert 1K-FA

> So, please do not advise someone with a solid state amp to disconnect 
> their ALC- unless you want to assume the cost of repairs.

Overall, I agree with that assessment.  However, I do no not make use of the

K3's ALC line with my SPE solid-state amp.  In looking at the SPE's 
protection circuitry, a fault buss is created that applies a high level of 
bias to the power MOSFETs when any of about 6-7 different fault conditions 
occur.  The response time is exceptionally fast to the point where if I 
deliberately blast the SPE with full power from the K3 without ALC connected

(don't try this at home - I'm a risk taker but did so only after reviewing 
the circuit), the SPE detects the presence of high input power and applies 
enough MOSFET bias to keep output power below about 1100 watts.  How 
instantaneous?  I have not measured the response time but it does respond 
well to the leading-edge power spike from another well-known transceiver. 
Today, a solid-state amp should be smart enough to know when it's being 
overdriven.

Also, an unknown with the SPE amp is what happens to amplifier linearity 
when the bias protection circuitry begins to engage to keep power from 
exceeding 1KW?  But the same question must be asked of any transceiver that 
interfaces to an amp while using an ALC line.  I like to think that when an 
amp and transceiver come from a common manufacturer, the ALC circuitry is 
probably optimized since the designer knows exactly what's needed at the ALC

"meet me" point between the amp and transceiver.

Paul, W9AC 

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