[Amps] Tuned inputs

[Roger]

Dave White wrote:
> 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.
>   
This leaves me with the question as to why it acts that way when the one 
tube does not draw grid current so I'd think the SWR and impedance would 
be constant.  With auto tune off, the SWR appears to be very close to 
1:1 as the indicator does not move off the peg.

73

Roger (K8RI)
> 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 at texascellnet.com>
> To: "'DAVE WHITE'" <mausoptik at btinternet.com>; "'Roger'" <sub1 at rogerhalstead.com>
> Cc: "'Jim Thomson'" <Jim.thom at telus.net>; amps at 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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