[Amps] New Guy, Old 813 amp

Sat Apr 28 01:14:42 EDT 2007

Possibly because the unterminated input meant that feedback (i.e. output reflected back to the input port via the tube capacitance) was having a much greater effect than with a lower impedance tuned input. 

73
Bob, WA2CKY

----- Original Message ----
From: Robert Bonner <rbonner at qro.com>
To: k7rdx <k7rdx at charter.net>; Roger D Johnson <n1rj at adelphia.net>; amps at contesting.com
Sent: Friday, April 27, 2007 8:19:11 AM
Subject: Re: [Amps] New Guy, Old 813 amp

Interesting Jim.  That's cool that the tuner worked for you.  When I banged
together my single band PLASMATRON the pair of 8877's for 80 meters a few
months ago, I hadn't built an input network for it yet.  The input was 3:1
and would tune with the tuner in the 7800.

I did notice the complete swinging SWR/tuner hunting that Tom mentioned.

Now I wonder.  Not ever truly measuring the cathode impedance and all...
But having a 20 foot long hunk of LMR400 running from the rig to the amp
could be quite the impedance transformation device with a mismatched load on
the other end.  Who Knows what the rig is actually seeing?

Fortunately I only did this for a night and the second day I whipped up a
network and had it all under control, no longer needing the tuner.

Once the Input is tweaked in, I don't see the swr changing with
current/load.  SO why is that?  If the input impedance is supposed to go all
over the place why does it NOT once you install a network?

BOB DD

-----Original Message-----
From: amps-bounces at contesting.com [mailto:amps-bounces at contesting.com] On
Behalf Of k7rdx
Sent: Thursday, April 26, 2007 11:06 PM
To: Roger D Johnson; amps at contesting.com
Subject: Re: [Amps] New Guy, Old 813 amp

The LDG amp module is easy to install and use in a HB amp. The purists will
all agree against using an L network for cathode matching and I tend to
agree to some extent. I bought an LDG amp module for my 3CPX5000A7 project
and so far have had much success with it.This tube has a cathode impedance
of 25 ohms and feeding the cathode directly results in nearly 2 to 1 SWR.
The LDG tuner module easily matches my rig to the amp with less than 1.2
SWR and does so with ease. No,it doesn`t cycle all over the place looking
for a match..When I change bands it`s memories find the best setting within
a few seconds or less,a few watts drive when first tuning up and it`s
there.Fully automatic.The maximum drive power LDG recommends is 125
watts...My amp has been stable since day one.Do I have a spectrum analyzer?
No,but I have tested with a modest Kenwood scope and the output looks really
clean and on the air reports seem to confirm this too.Will I use the LDG in
my next amp? I don`t know...It was new to the ham market when I built this
amp and I decided to try it. For what it`s worth,it`s worked for me,73 all,
Jim K7RDX..
----- Original Message ----- 
From: "Roger D Johnson" <n1rj at adelphia.net>
To: <amps at contesting.com>
Sent: Thursday, April 26, 2007 11:29 AM
Subject: Re: [Amps] New Guy, Old 813 amp

> Doug wrote:
> > Here is the link, it is a product from LDG.
> >
> > http://www.ldgelectronics.com/products.php?cID=7&pID=2&v=1
> >
> > 73, Doug
> > N9XTF
> >
> >
> I have a bit of heartburn with this scheme. The LDG tuner is an L
> network. The
> Q of an L network is not independently selectable. It depends on the
> ratio of
> impedances to be matched. For example, to match the 135 Ohm impedance of
> the pair of 813s under discussion, the L net would have a Q of only 1.3.
> This is
> far less than the 2 usually used in older amps or the 3 to 5 recommended
> for solid
> state drivers. It gets worse as the input impedance of the tubes
> decreases. For
>  instance, a pair of 3-500Zs with an input of 60 Ohms, would result in a
> Q of only
> .45!
>  Another problem I see is that with a GG amp, the input and output are
> in series.
> With my old Drake L-7, if the output is mistuned, the input SWR is high.
> This
> causes my FT-1000 to throttle back the drive power automatically. As the
> output
> of the amp is tuned, the input SRW decreases and the drive power comes
> up. This
> make for an easy tuneup procedure. With an autotuner on the input, it's
> going to be
> constantly chattering looking for a match as the amp is tuned. Perhaps
> it can be
> programmed to come up in the preset mode rather than the auto mode.
>
> Good idea....needs work.
>
> 73, Roger
>
> -- 
> Remember the USS Liberty (AGTR-5)
> http://ussliberty.org/
>
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