INTRODUCTION TO ELECTRON TUBES

TETRODE CHARACTERISTICSBecause the screen grid is in the electron stream, it will always draw some current. The currentdrawn by the screen grid will be lost to the plate. This means that the transconductance of a tetrode, whichis based on the amount of plate current versus control-grid voltage, will be lower in tetrodes than intriodes. The formula for transconductance of a triode,must be adjusted for screen-grid current, and becomesAs you can see, the transconductance for a tetrode can never be as high as that of a triode of similarconstruction.While lowered transconductance in a tetrode is an undesirable characteristic, it is not the reason thattetrodes have found little acceptance in electronics. The factor that severely limits the operation oftetrodes is SECONDARY EMISSION.Because the screen grid is positively charged, electrons traveling from the cathode to the plate areaccelerated. Electrons are accelerated to such an extent that they dislodge electrons from the plate whenthey strike it. This is similar to the manner in which a high-velocity rifle bullet fired into a pile of sawdustthrows sawdust about. Some of these electrons are fired back into the tube, where they tend to accumulatebetween the screen grid and the plate. This effect is most pronounced when the signal at the control grid isgoing positive and conduction through the tube is increasing. The plate in this situation is going negativein answer to the control-grid signal. This causes the electrons accumulating between the plate and screengrid to be attracted to the screen grid. The current that is drawn by the screen grid is lost to the plate andgain is decreased. Gain is also decreased in another way. The negative charge accumulated by secondaryemission causes some of the electrons (from the cathode) to be repelled from the plate, which furtherreduces gain.Another undesirable characteristic of tetrodes associated with secondary emission is that the outputsare NOISY. What this means is that small sporadic signals appear on the main output signal, as shown infigure 1-32. When electrons are knocked from the plate, they represent losses of plate current andcorresponding positive pulses on the output. Electrons falling back to the plate represent increases in platecurrent and cause negative-going pulses to appear in the output.1-46