In practice the negative-resistance kink of the anode characteristic limits the usefulness of the screen-grid valve to applications where only small signals are amplified, so that the variations in anode voltage are also correspondingly small, and the operating conditions never depart from the region of high positive impedance (low positive slope) at large anode voltage. The secondary emission can be prevented from contributing to the screen current (and thus detracting from the anode current) by adding a suppressor grid, making a pentode, or beam plates to make a beam tetrode/kinkless tetrode, which can be used in power amplifiers where the variations of anode voltage, due to the presence of a large signal voltage, are much greater. The anode characteristics of these tubes are very similar to pentodes. In both cases the anode current rises rapidly to an approximately constant value when the anode voltage rises to as little as a few tens of volts, and the region of high positive anode resistance (low slope) extends from this low value up to the maximum anode voltage which could be several hundred volts or more.