"Alejandro Lieber" wrote in message
...
Alex Pogossov wrote:
"Alejandro Lieber" wrote in message
...
Very interesting. Do you know if it has been done in practice ?
Will the final valve still work in class C all the modulation cycle ?
I also remember seeing a rig using 2 6W6 (or 6Y6) in parallel in the
cathode of the RF output valve.
Cathode modulation has more advantages.
1. It is a sum of control grid modulation and partly screen modulation.
As
the cathode voltage rises at the troughs of modulation, voltage
difference
between the screen and cathode reduces. Thus the "pull" of the screen
grid
reduces which cooperates with the effect of the cutting of by the control
grid. Thus the cathode voltage excursions are smaller than control grid
excursions if it were purely grid modulation.
2. Modulation is not affected by variation of excitation voltage. In a
common grid modulation, if the excitation voltage say is modulated by
50Hz
hum, output will also be modulated. With the proposed cathode current
modulation output will not be affected. Cathode voltage might be
modulated
by hum, tracking it like in a cathode detector, but since the current is
stable, the anode RF component will be almost unchanged (ignoring cut-off
angle effects).
But the greatest advantage is no need for any adjustments of bias, etc.
The
only thing to be set is the cathode current. It is done by adjusting bias
on
the gate (base) of the modulator series transistor.
Very interesting.
Where can I look for a practical circuit ?
Nowhere. The whole fun is to create this circuit and experiment. Patrick has
done similar thing (cathode current modulation) in his low distortion test
AM modulator. You have a chance to try in a higher wattage range. I did it
many years ago with a smaller tube, similar to 6BW6, for a 10m band
transmitter.
Regards,
Alex
--
Alejandro Lieber LU1FCR
Rosario Argentina
Real-Time F2-Layer Critical Frequency Map:
http://1fcr.com.ar
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