-----Original Message-----
From: Gary Schafer [mailto:garyschafer@comcast.net]
Sent: Wednesday, April 21, 2010 11:19 AM
To: 'Dave White'; 'Phil Clements'; 'Roger'
Cc: 'Jim Thomson'; 'amps@contesting.com'
Subject: RE: [Amps] Tuned inputs
Nothing isolates the varying input impedance of the tube.
Even with a tuned input network the swr will vary depending on the tube
operating parameters. The input matching network needs to be optimized at
full power of the amp.
I have found using an auto tuner into an amp that does not have a tuned
input that if you load up the amp to full power and activate the autotuner
while still at full power it will tune properly and stop. Disable the
autotuner so it will not retune. Then re-peak the amp again and re-enable
the auto tuner, let it tune until it stops again. Disable the tuner again
and you are good to operate. The tuner will be optimized for the amp at full
power and the exciter will be matched to the amp at full power.
If you run the autotuner at lower power levels things will not be matched at
full power where it is needed.
73
Gary K4FMX
> -----Original Message-----
> From: amps-bounces@contesting.com [mailto:amps-bounces@contesting.com]
> On Behalf Of Dave White
> Sent: Wednesday, April 21, 2010 10:38 AM
> To: Phil Clements; 'Roger'
> Cc: 'Jim Thomson'; amps@contesting.com
> Subject: Re: [Amps] Tuned inputs
>
> Thanks Phil. I can identify with the tuner whose job is an exercise in
> never-ending futility....I know how it feels....
>
> Presumably, then, the un-un is just that: an impedence transformer, but
> one without a flywheel effect and whose impedence will change with that
> of the tube.
>
> Also, I presume that the tuner is isolated from the varying input
> impedence of the tube by the tuned input circuit...
>
> Makes sense to me.
> Cheers
> Dave
>
>
> -----Original Message-----
> From: "Phil Clements" <philc@texascellnet.com>
> To: "'DAVE WHITE'" <mausoptik@btinternet.com>; "'Roger'"
> <sub1@rogerhalstead.com>
> Cc: "'Jim Thomson'" <Jim.thom@telus.net>; amps@contesting.com
> Sent: 20/04/2010 23:24
> Subject: RE: [Amps] Tuned inputs
>
>
>
> > Roger, I've never personally had that problem with anything other than
> the
> > 3CX3000 amp with that Henry un-un in the circuit, so put it down to
> some
> wierd
> > effect from the un-un. Both the FT1000 and IC7800 do the same thing
> but
> are fine
> > into other non-perfectly-matched loads. Theoretical explanation,
> anyone?
>
> Henry 8K/3K Ultra Operating Manual; page 40:
> "With some transceivers, particularly those with built in antenna
> tuners, we
> have found that some of them cannot get a good match in AUTO mode. If
> you
> turn off the automatic antenna tuner, and run the transceiver directly
> to
> the 3KU or 8KU, you should have no problem."
> This should be obvious why the AUTO tuner does not work; unlike a fixed
> load, as an antenna or dummy load, the input impedance of the tube is
> constantly changing during operation, and the tuner searches for a match
> every time the impedance changes. This is like a dog chasing its tail.
> Henry
> guarantees an input vswr of 2.0:1 or less. Typical is 2.0 to 1.8:1. The
> tuner on AUTO is constantly trying to seek a 1.1- 1.5:1 vswr. By the
> time it
> succeeds at this, the tube input impedance has already changed to
> another
> value, and away the tuner goes again, trying to do its job; a never-
> ending
> exercise in futility.
>
> Your experience with other "non-perfectly matched loads" is just a
> matter of
> luck!
>
> (((73)))
> Phil, K5PC
>
>
>
>
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