On Wednesday, 10 October 2012 10:39:54 UTC+11, Alejandro Lieber wrote:
I am thinking in building an AM transmitter for 7.1 Mhz (40 meters) using a EL34 in class C as final. The idea is to modulate it by varying the negative polarity of the suppressor grid (g3). Suppose I load the EL34 plate with 500 volts DC and a current of 150 ma with 0 volts suppressor, does anyone have any idea what negative voltage in g3 is needed to reduce the anode current by half, needed for screen AM modulation ?. Alejandro Lieber LU1FCR Rosario Argentina Real-Time F2-Layer Critical Frequency Map: http://1fcr.com.ar --- news://freenews.netfront.net/ - complaints: ---
Methinks an 807 would be a lot better because 807 has an anode top-cap connection rather than to pin 3 of an octal socket, from where arcs to pin 2 which is the filament usually at earthy potential.
AFAIK, suppressor grid 3 is a bit non linear for modulation on its own. But in you post you wonder what's needed for "screen AM modulation" which I think is much more linear. You should br able to find the set of curves for Ia versus Eg2amoung characteristic curves for EL34 or 807. Usually you will need quite a high voltage swing of maybe 150Vrms for considerable modulation %. And the screen draws DC current so its input impedance is not as high as for a negatively biased grid for AF. Presumably some RF current will be absorbed by the screen. There is plenty info in old UK and USA amateur handbooks on screen modulation. Maybe you will find you will need a another EL34 acting as an SET for audio amp to drive the RF amp screen, i'd guess you'd need a 20H choke feed to anode of audio EL34 and this anode could be directly connected to RF screen with Eg2 supply to choke at say 350V. I'd guess you would need a bypass cap from RF tube screen to 0V so that it operates as a pentode at RF but at AF the bypass cap has no effect. Maybe 200pF might be OK, its reactance at 7MHz = 113 ohms, but at 5kHz its 159k ohms.
Patrick Turner.