[Elecraft] ALC: issues raised almost 2 month ago + answer from Elecraft

[David Wilburn]

I admit, I may not be the sharpest knife in the drawer, but to see a
reference to an amp, and the words 'full power' and RTTY, all together?
Wow.  That raised my eyebrows.  

With the power density of RTTY, isn't that a full blown stress test of
your complete RF system, at 100% duty cycle, or so...?  Any standing
waves on the thing would be huge, and wouldn't the safety factor would
be a negative number?

Aren't you taking on a bit of risk to your system?  Please don't take
the wrong way, I am trying to learn.  Being one of those no-code techs
we hear so much about.  I am just rationalizing the described activity,
with all these warnings I see on various equipment, about high duty
cycle modes.
-  

David Wilburn
k4dgw at k4dgw.com
K4DGW
K2 S/N 5982


On Mon, 2007-12-03 at 16:43 -0500, Jerry Flanders wrote:
> At 02:59 PM 12/3/2007, Enno, PF5X wrote:
> >Hi gang,
> >
> ><SNIP> A very
> >quick response from Wayne gave the following info. Sufficient to cool the
> >discussion for a while I would say ...
> >
> 
> Not so - Wayne's comments seem directed at those who might want to 
> use ALC as if it were a substitute for the mike gain control.
> 
> It worries me when I see the discussion wander away from amplifier 
> safety in an antenna catastrophe situation, which is my main concern. 
> I once blew up the output circuit of an AL1200 due to an intermittent 
> in an antenna coax while running full power on RTTY.
> 
> Please see my other in-line comments below.
> 
> >--
> ><quote>
> >
> >From: wayne burdick n6kr at elecraft.com
> >
> >Date: Fri, 12 Oct 2007 14:12:24 -0700
> >
> >To: elecraft at mailman.qth.net, cw at dx.nl
> >
> >Subject: regarding external ALC
> >
> >
> >
> >Hi Enno,
> >
> >Transmit power control via the ALC input (AUX I/O pin 15) is planned, but
> >not finalized. We're mindful of the negative impact ALC can have on transmit
> >signal purity, a subject of recent debate within testing organizations.
> 
> Speaking only for myself, I do not intend to adjust the ALC to 
> provide normal level control - only a crisis shutdown function. 
> Quoting from my Quadra's manual p4.:
> 
> "The ALC cable must be connected between
> the transceiver and VL-1000 to
> prevent overdrive of the amplifier, and especially
> to facilitate the proper operation of the
> protection circuitry if sudden antenna system
> problems should cause the SWR to rise to dangerous
> levels."
> 
> Rightly or wrongly, Yaesu (and probably ICOM) engineers have presumed 
> the ALC would be functional and could cut back exciter drive in the 
> event of an antenna catastrophe. Other main-line rigs that I am 
> familiar with are compatible. I think it is reasonable to assume the 
> world-class K3 would have provided for this also.
> 
> BTW - It is worrisome that ALC was only planned, but still not 
> finalized, as of Oct 12.
> 
> <snip>
> >  Once ALC is working to our satisfaction, we'll provide simple interfacing
> >information that will allow the use of negative control voltages ...
> 
> I, and perhaps other Solid State amp users, would prefer a definite 
> calendar commitment, if that is possible. If not possible, please 
> state so. I would hope for a working, properly engineered 
> negative-going ALC fix within a few months.
> 
> >Meanwhile, the K3's DSP-based early-stage ALC is very effective at
> >preventing amplifier overdrive, and should suffice in nearly all cases.
> 
> I repeat -  my concern is amplifier safety, not a substitute for a 
> mike gain control. Sooner or later, the antenna will fail, but we 
> don't know when,  and we probably cannot manually cut back drive 
> quickly enough to protect those expensive SS finals when it occurs.
> 
> >73,
> >
> >Wayne
> >
> >N6KR
> 
> 
> Jerry W4UK
> 
> 
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