[Amps] TUNER

[Jason Hissong]

I was given a really nice homebrewed amp last year that works well,
but is missing a tuned input.  I used to use it with my TS450SAT and
it seem to worked pretty well with it.  The 756Pro3 has issues with
it.  I have pretty much decided to not use my G-G amp anymore with my
756Pro3 until I get a tuned input network in the amp.  I was lucky
that no antenna tuner was harmed in the process (well, they appear to
work normally while barefoot anyway).

The amp uses 4 572Bs operating in parallel.  I was looking at the
tuned input network in an SB200 schematic and was wondering if that
would do the job.  The bands are exactly the same (10,15,20,40,80).
Is this a good place to start?

I need to check the bandswitch on my amp and see if there is an extra
wafer I can use for switching the input network at the same time I am
switching the output network.

When I start this project, I am pretty sure I will have many questions.

73, Jason N8XE

On Mon, Oct 11, 2010 at 9:43 AM, Jim Thomson <jim.thom at telus.net> wrote:
> Date: Sun, 10 Oct 2010 07:03:52 -0700 (PDT)
> From: Bob Gibson <w5rg at yahoo.com>
> Subject: [Amps] TUNER
>
>
> If you put the tuner at the back of the Amp..Why not just use the tuner in the
> Radio? Bob
>
> ## You folks keep forgetting the cathode RF return current pulses  are via the C2
> cap of any  PI tuned input.   Those pulses are typ  3 times the plate current for
> AB-2... and  4  times the plate current for class C.   I already know one guy here
> in town who has fried the auto tuner in his FT-990..from using it as a tuned input.
> IE: the auto tuner has to hanle the xcvr's normal RF.... PLUS the 3 x times plate current pulses.
> IF your built in PI net tuned circuits  in your amp have a bit of swr on band edges
> then it's ok to use the auto tuner in the xcvr to take the swr down to 1:1, otherwise,
> don't mess with the auto tuner.  Besides, when you switch to barefoot, the auto tuner
> now see's the ant swr, and has to re-tweak itself.
>
> ##  If you mount the tuned input any distance from the cathode, eff  will  go to hell
> really fast.   You want the C2 cap as close to the cathode as practical.  Putting
> a small fixed cap right at the cathode will shift some of the C2  directly to the
> cathode, increasing eff on the higher bands... but it can't be too high, otherwise
> you won't be able to adjust 10m.  50-150 pf  NPO is usually ample.
>
> ## another trick is to use < 50 ohm coax  between C2  and the cathode, esp if
> the coax length is > 12-14".   2 x RG-59's in parallel...or  2 x RG-58's  will  do the
> job.  That's in effect,  like moving C2  closer to the cathode.  That  works good on the
> upper bands.
>
> Jim   VE7RF
>
>
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