The high value of the anode slope resistance of tetrodes (mentioned above) makes them capable of high voltage and power gain, and is also potentially a cause of high anode efficiency which, if it could be exploited, would make tetrodes superior to triodes as power amplifying devices in applications such as audio power amplifiers, and the output stages of radio transmitters. For a triode power amplifier working with a transformer or inductive load in Class A, the maximum theoretical efficiency is 25%. This low figure is in part a consequence of the low anode slope impedance (R) of this type of tube; the low value of a triode R is almost always much less than the optimal anode load impedance in a power amplifier. For a pentode or tetrode, however, R is usually sufficiently high for the optimal load impedance to be achieved, and under these circumstances the maximum theoretical efficiency rises to 50%. This gives tetrodes and pentodes an important practical advantage over triodes, which is of particular value when high power outputs are required.