Re: [Amps] PARALLEL CAPS IN OUTPUT

["Carl" <km1h@jeremy.mv.com>]

----- Original Message ----- 
From: "Jim Thomson" <jim.thom@telus.net>
To: <amps@contesting.com>
Sent: Friday, December 06, 2013 1:54 PM
Subject: [Amps] PARALLEL CAPS IN OUTPUT


> Date: Fri, 6 Dec 2013 10:21:59 -0500
> From: "Carl" <km1h@jeremy.mv.com>
> To: "Jim Thomson" <jim.thom@telus.net>, <amps@contesting.com>
> Subject: Re: [Amps] PARALLEL CAPS IN OUTPUT
>
> > ##  For folks with a PI net, make sure you use a piece of 50 ohm coax
> > between the load cap and the TR relay.
> > And with shield bonded to chassis at BOTH ends.   If wire or cu strap is
> > used instead...then u end up with a huge Z bump.
>
> **  The coax isnt long enough to act as a transmission line at HF....in 
> most
> amps anyway.
>
> ####  OK, do it ur way.  Use a chunk of bare wire, strap etc..and watch 
> the
> PI net load cap values change asap.   Ditto with coax...and only 1 end of 
> shield bonded to
> chassis.
>
> ###  strap, wire, etc is NOT 50 ohms, not even close.  Try it urself.
>
>
>
> > That alone will put the load cap at something higher than a 50 ohm
> > point... increasing the peak V.  On 10m band, the
> > stray L  of a piece of wire instead of coax  will make the PI  now appear
> > as a PI-L.... putting the load cap at a much
> > higher Z point.   same deal peak V increases.     Now with a wide open on
> > the ant, coax cable etc, the load cap
> > will weld shut.
> **  All the coax does is act as a shielded cable with some value of C.
> At 10M even 1/10 wave of cable is 1.6' so it is invisible as coax.
>
> #####  I don’t believe it for a second.  Of course, I have  tried it both 
> ways.
> Replacing the junk typ used from load cap to TR relay   will solve..and 
> has
> solved all sorts of issues.
>
> Later....... Jim   VE7RF
With all your bluster you still dont get it. All you are using is a short 
piece of shielded cable, impedance has no bearing since it is too short to 
present a meaningful reactance.
Your load cap change is due to the added C to ground of that shielded cable.

Having it is beneficial if the cable is long or passes near other circuitry 
such as the input. Lift the RG-8 shield on a SB-220 and see what happens.
A high percentage of the ones I get thru here have cold solder joints which 
will do wonders for stability.
Carl 

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