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Re: [Amps] XCVR Output vs. Amplifier Input Impedence

To: "'Wayne Rogers'" <>, <>
Subject: Re: [Amps] XCVR Output vs. Amplifier Input Impedence
From: "Gary Schafer" <>
Date: Sun, 9 Mar 2008 15:43:12 -0400
List-post: <">>
The source impedance (output impedance of the transceiver) has no bearing on
the input SWR of the amplifier. Anything that you do to the source impedance
won't change the SWR at the amp input circuit.

Keep in mind that the output impedance of the transceiver changes with power
level. Just like when you tune the plate and load tuning on your amplifier,
if you change the power level (drive) the tune settings are no longer
optimum because the final tubes present a different load impedance at the
new power level.

As to GG input networks, changing only one element like the coil inductance
will only change the center frequency. It will have very little if any
effect on the matching impedance at either end. Being able to vary at least
2 elements are desirable. You can use a slug tuned coil and tack a trimmer
on each end and when you get the desired match measure the variable caps and
replace them with fixed caps.
The input impedance will be different depending on the power level that you
decide to run the tube at.

Gary  K4FMX

> -----Original Message-----
> From: [] On
> Behalf Of Wayne Rogers
> Sent: Sunday, March 09, 2008 1:38 PM
> To:
> Subject: Re: [Amps] XCVR Output vs. Amplifier Input Impedence
> This discussion about the output impedence of a solid state transceiver
> has
> some bearing on my current problem.
> I am beginning to build a 4-1000A G-G linear (I know - old technology, but
> thems the parts I have).  I am trying to figure out the best approach for
> coming up with an input network.  My research to date says that the
> impedance varies snywhere from around 25 to 200 ohms, frequency dependent,
> and I would guess is dependent on some of the input components (like the
> bifilar choke on the filaments).
> So how do I approach the design?  I am planning on a pi-network with a Q
> of
> 3 or so.  The values are easy to calculate if I know the tube input
> impedence, but I don't.  I could design for 100 ohms and hope for the
> best?
> Also, with a  low Q I'm expecting to have only one of the three pi-network
> components variable, but which one?  What has your experience been?  I am
> building this amp for the low bands - 160 - 40, so I won't have to deal
> with
> the problem of making a 4-1000 play on ten meters.
> Any input appreciated.  Thanks in advance.
> Wayne N1WR
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