A simple question seems to have been the hot button for a good number of
hams.
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.
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.
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.
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.
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.
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.
But, if you want to get at least one ham really wound up, just mention the
circuit.
Colin K7FM
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