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Suppressor-grid modulation
"Antonio Vernucci" wrote in message
... Hello Tony, de Piero I5SPO. Quite a ( very ) long time ago, i've done an A.M. TX with a Geloso VFO and a class C RL12P35 Wehrmacht pentode, whose characteristics are similar to 1625 ( a.k.a. 807 with 12.6 v filament ), using a carbon mike and a step-up audio transformer. I believe that, using beam tetrodes, there's no way... :-( I know also 813 and 814 ( have 2 pcs ) and these, like 807, are useful for controlled carrier A.M. by modulating screen grid. Do not know 803, but if i remember well, are low power tubes. Used with suppressor modulation, their output will be one fourth of CW rated output, and efficiency is consequently low... :-( HTH, 73, Piero. Hi Piero, the 803 is about the same as the 813, though it has a proper suppressor screen. Other differences are socket (5 pin vs. 7 pin) and maximum frequency (20 MHz against 30 MHz). What I need to know is if any one has ever tried to vary the 813 beam forming plate voltage, so as to verify whether the plate current can so be controlled. If so, I could avoid to purchase the 803s and use the 813s I already have. I know that with suppressor screen modulation output power is about one fourth, but efficiency will not be so low because, in absence of modulation, also the input power decreases (by somewhat less than one fourth). With a pair of 803s (or 813s I hope!) I should obtain about 200W of unmodulated carrier power, peaking at 800W under 100% modulation. I used to have a few RL12P35s, though I never actually used them. A Radio Rivista article by Dante I1DC describes a transmitter using the RL12P50 (a bigger brother of the RL12P35). But all those tubes are too small for well exploiting the potential advantages of suppressor screen modulation. As a matter of fact the advantage of not having to build a powerful modulator becomes more evident when the needed modulator would be real big! 73 Tony I0JX True pentodes are needed for suppressor grid modulation. The beam forming plates have much less of an effect on plate current and trying to use them for modulation will result in a highly nonlinear modulation waveform. Remember that emission current not going to the plate will instead go to the screen grid. The power dissipation of the screen grid is what seriously limits the use of suppressor modulation. If you already have the sockets for the 813, I would suggest the use of 4E27A/5-125B tubes. The 4E27/Heintz & Kaufman 257B has less plate dissipation, but it too might be suitable. Please let me know if you need datasheets for either of these tubes and for the 803. 73, Barry WA4VZQ wa4vzq(-at-)live(-dot-)com |
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