[AMPS] G2DAF Circuit

[Radio WC6W]

On Wed, 25 Oct 2000 22:05:21 -0700 "Lamb" <k7fm@teleport.com> writes:

Hi Colin,

>
>A simple question seems to have been the hot button for a good number of
>hams.

    That is puzzling.

>When the issue came up some time ago, I breadboarded a beam power tube
and
>monitored the screen and control grid voltages with plate current, since
I
>could not understand how such a circuit could work.

   You built one!?   I had a talk with a pair of 6X4's a while back and
they told me to close their box tops and let them go back to sleep. :-)

   It does appear that the laws of the universe are being trifled with
upon initial inspection.

>After carefully plotting control and screen voltage and current with
plate
>voltage, I now have a theory of how the circuit could work.  Linearity
would
>be obtained if the inefficient screen voltage doubler circuit causes the
>screen voltage increase to lag behind the control grid voltage increase,
>with excitation, due to increased screen current.

   The screen voltage needs to follow the envelope of the SSB signal,
WITHOUT lag, for the circuit to work.

>Linearity would depend upon the screen charging fast enough to follow
the
>audio and a circuit designed for one tube would not necessarily work for
>another.  One would not want to build one without having sufficient test
>equipment and knowledge to determine whether it is operating with
acceptable
>distortion levels.

   Yes, it would be rather sensitive to the individual tube
characteristics.   That's why I recommended closing the loop on the
screen voltage yesterday to force the circuit to operate in a linear
fashion.

>Although the load would change with drive, I am not convinced that is as
big
>a problem as it might first appear.  When drive is less, the output is
less.
>Buckshot usually occurs at peaks, as amplifiers flatten out.  What is
more
>troubling is whether the exciter is properly loaded on both half cycles
at
>peak power.  That is the reason for using a tuned input in the grounded
grid
>circuit.  The same flywheel could be applied to the DAF circuit, as Marv
>suggested.  But, we could also have rectifiers of opposite polarity
>operating into a resistor to load the negative cycle without a tuned 
>input.
>

   The 300 ohm swamping R of the original should have been adequate to
keep the load constant combined with the flywheel action of the (tube
type) exciter's output network.

>Peter is using his own version of the G2DAF circuit on the air and gets
>favorable comments.  He publishes the spectrum analyzer results on a
>website.

    Peter has been... wisely quiet, during this quarterly revisiting of
the DAF concept.

>Many tubes would not be acceptable to try in such a circuit, and many
>modified circuits could not work.  If you want something simple or
>foolproof, try another circuit.

   As noted above, it isn't a reliably repeatable circuit, which helps to
prolong the controversy!

>But, if you want to get at least one ham really wound up, just mention
the
>circuit.

   Maybe, two!

73,
   Marv  WC6W


















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