On Saturday, 13 October 2012 16:12:47 UTC+11, Alex Pogossov wrote:
"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 --- news://freenews.netfront.net/ - complaints: ---
One small tube which might be good for G3 AM modulation could be the small mini 7 pin 6DT6, because it was common for an FM demodulator using quadrature detection, but fairly simple, and 6DT6 was made to have good G3 gm character.
Once the RF is AM modulated, it can just be amplified by a following RF output tube, maybe 1 x 807 is good.
Patrick Turner.