[Elecraft] (no subject)

[Edward R Cole]

Vic,

Thought a bit more after my private reply I sent you, yesterday.

Of course, I agree with you that using a simple RF sensed PTT is hard 
on amplifier and TR switch.

My reply was to a question how to build an RF sense ckt and not how 
to design one into an existing amplifier.
One usually finds these on VHF amps designed for FM.  I suppose they 
just suffer hot switching for the price they get.

In the cited W6PQL 2m amp, he actually addresses this by adding a 
bias delay ckt so the amp does not pass RF until TR relay has 
switched.  I suppose one could duplicate that, but if done on a 
KXPA-100 it would likely void any warranty.

Way better to use external PTT which is provided for the 
KXPA-100.  The K3/10 has min 8ms delayed RF upon PTT to provide 
proper sequence of operations.  I suppose the KX3 has a similar delay 
keying ckt.  How this affects QSK operation is beyond scope of the 
original question - which is all I answered.

But good that you pointed out the hazard of simply adding RF sensed PTT.

73, Ed - KL7UW

-----------------------
From: Vic Rosenthal <k2vco.vic at gmail.com>
To: Edward R Cole <kl7uw at acsalaska.net>
Cc: Elecraft at mailman.qth.net
Subject: Re: [Elecraft] K3/K3S 10W models with KXPA100
Message-ID: <DC2EBAAE-6180-4F2C-A313-C33B16F9041A at gmail.com>
Content-Type: text/plain;       charset=us-ascii

I don't like circuits like this because they guarantee hot-switching 
of the amplifier. Unless you have a time-reversal device, the amp 
won't switch until RF is already flowing. Maybe it wouldn't damage 
the amplifier in this case, but it would certainly mess up the 
shaping of CW elements.

Much better to add an amp key circuit to the transceiver if it 
doesn't have one. If done correctly (like the N0SS circuit for the K2 
that was mentioned), the amp key line is activated a few ms before RF 
is produced.

Vic 4X6GP



73, Ed - KL7UW
http://www.kl7uw.com
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