Bob,did you use a toroidal balancing transformer before the coupling caps
and cathodes?(Like Alpha 77sx) A pair of 8877`s must look close to 25 ohms
at the input.The LDG module will cycle if more than say 10 watts is used for
initial tune up...After tuning is stored in memory,you can qsy right back
with full drive and a match is found very fast..I have less than 6 inches of
coax between the module output and tube cathode coupling caps and the
module(Seperate enclosure) is protected from rfi..I can put my 950SDX meter
in swr mode and watch the amp input as I operate...Very stable. BTW,I knew
abt the filament hum problem Tom mentions....You can`t use ct on the fil
trans for neg return with a tube that has one side of fil and cathode
common..Found that out (& cured it)several years ago while playing with a
GS-35b..73,Jim..
----- Original Message -----
From: "Robert Bonner" <rbonner@qro.com>
To: "'k7rdx'" <k7rdx@charter.net>; "'Roger D Johnson'" <n1rj@adelphia.net>;
<amps@contesting.com>
Sent: Friday, April 27, 2007 6:19 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@contesting.com [mailto:amps-bounces@contesting.com] On
> Behalf Of k7rdx
> Sent: Thursday, April 26, 2007 11:06 PM
> To: Roger D Johnson; amps@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@adelphia.net>
> To: <amps@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/
> >
> > _______________________________________________
> > Amps mailing list
> > Amps@contesting.com
> > http://lists.contesting.com/mailman/listinfo/amps
>
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