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@telus.net> wrote:
> Date: Sun, 10 Oct 2010 07:03:52 -0700 (PDT)
> From: Bob Gibson <w5rg@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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