[AMPS] Re: Autotune amps & the 2uH thingy

[Radio WC6W]

On Fri, 30 Oct 1998 02:48:15 -0500 km1h@juno.com writes:
> Has there been anything published on a real autotune amp? I do not mean
a
>bandpass design such as the old Alpha's, etc.  I do not want the 
>problems of the early Collins either ( I worked on the ARC-27 !!) but 
>how about a cross between the old and the new. Ideally it
>would be controlled by a PC, not an on board processor..
>Just getting ideas for the YC-156.

Hi Carl,
   I found that RG-402 trick just as interesting when I first saw it.  
:-)

   I am currently involved in an update of my own ridiculous amplifier
and I may just drop one of those coils in for the sake of science.

   Switching to autotune ...

   The same Hughes piece where the coil resides is fully autotuned.  And
they also used the same techniques in a number of multistage /
multikilowatt amplifiers of the early '60's.

   The basic layout had individual tuned grid - tuned plate stages with
each individually neutralized plus some of the higher power models
employed RF feedback around the last two stages to reduce distortion.  

   They employed a diode descriminator to sense proper tuning with the
input coupled through a little toroid from one leg of the grid tank and
the reference signal injected via a capacitive divider from the plate
circuit.   The theory is that when the stage is in tune there will be a
180 degree difference between the plate and grid signals.  By picking off
the input from one leg of the input tank you get a free 90 degree phase
shift at resonance which makes the descriminator output zero when the
stage is in tune.

   There were a number of presets selected by sensing the input frequency
to force the tuning fairly close before enabling the autotune servos so
as to allow reasonable range & gain in the servo amps and also avoid the
obvious problem of harmonic locking.  

    They also have a loading comparator circuit, similar to that employed
in a number of  Collins designs ( the 30S1 comes immediately to mind),
for sensing and adjusting the load capacitor via servo.  

   All of these servo amplifiers were of chopper design, likely directly
using the 400 Hz power as the switching rate in the airborne units.

   I think that all of these techniques can be found in the professional
literature of the '50's if you dig deep enough!   The only amateur
reference I can recall would be in "Amateur Single Sideband" by Collins
Radio, 1962.  They describe the tuning & loading methodology in chapter
4, sections 10 B & C respectively.  

   I'm not familiar with the ARC-27... is this the same scheme?

   In my 4CX1500B linear, built around 10 years ago, I have stepper
motors, driven by a sine drive under microprocessor control, direct
driving the vac. variables and roller coils of the Pi-L network.   I
didn't do anything fancy with the autotune, at that time, beyond
including about 100 presets, which were selected by the amplifier as the
result of interrogating my Kenwood 440 to ascertain its transmit
frequency, and then just tweaking the plate cap automatically for the
fine tune.  

  As part of the redo, which mostly concerns the software, I am
installing the sensors to try the "true" autotune as detailed above.   

  I did have to surmount one minor problem in using that scheme.  Since
my amplifier has a swamped grid input, I didn't have the free phase shift
available to use the diode descriminator.  Therefore, I am employing a
modern phase comparator, using a couple of CMOS parts, that can directly
resolve the grid & plate signal's phase differences.

  I will also be adding a direct frequency count to the input, in case I
ever need to use the amplifier with an exciter besides the Kenwood.  

73 & Happy Halloween,
   Marv  WC6W






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