Date: Tue, 20 Apr 2010 17:24:54 -0500
From: "Phil Clements" <philc@texascellnet.com>
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!
## I have found that both my yaesu MK-V.. and also my 1000-D will
match [auto tuner on] any GG linear that does NOT have a tuned input
at all, including single 4-1000 [100 ohms] and also 813 gg amps,
MLA-2500 amps [ no tuned input version].. and single 3-500Z [130 ohms]
## I also found, with way too long a coax jumper is used, between my
1000-D and input to L4B... that input swr rises. The auto tuner match's
just fine. I also tried [temp] gross maladjustment of tuned input slugs
on the L4B on a few bands... and again, the auto tuner will easily match the
2:1 or even 2.5:1 swr just fine.
## I dunno what it is with the 8-K ultra broad band un-un set up...
but it sure HATES auto tuner's... and the auto tuner in the 1000-D will
match some pretty bizzare loads. Some xcvrs'... like my 1000-MK-V..
will throttle back the power output.. a whole bunch, with a 1.8:1 swr.
## with a real... PI tuned input on a 3x3, drive requirements are way down,
and IMD is reduced a bunch. Input swr is dead flat...and xcvr is ..'happy'.
## IF the 8 k un-un set up is a dead loss on the 50 ohm input Z of a 3x3 tube,
I would suspect it would be even worse with a 100 ohm input Z tube..or
a 25 ohm input Z tube. There is NO flywheel action at all with the un-un
setup, tank eff suffers, po drops off a whole bunch, imd suffers. I'd avoid
the un-un as a ..'tuned input' like the plague.
later... Jim VE7RF
(((73)))
Phil, K5PC
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