Advantages Over The Tetrode
A tetrode could supply sufficient power to a speaker or transmitter, and offered a larger amplification factor than the earlier triode. However, the positively charged screen grid can collect the secondary electrons emitted from the anode, which can cause increased current toward the screen grid, and cause the anode current to decrease with increasing anode voltage over part of the Ia/Va characteristic.
A pentode, as introduced by Tellegen, has an additional electrode, or third grid, called the suppressor grid that solves the problem of secondary emission. The suppressor grid does this by being held at a low potential, usually either grounded or connected to the cathode. Secondary emission still occurs, but the electrons that come from it can no longer reach the screen grid as they have less energy than the primary electrons, and, hence, cannot pass the grounded suppressor grid. The electrons from the secondary emission are re-collected by the anode.
Pentodes, therefore, can have higher current outputs and a wider output voltage swing; the anode/plate can even be at a lower voltage than the screen grid yet still amplify well.
Read more about this topic: Pentode
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