Let me put on my 1960's tube hat for a minute....
A triode has one grid, called the control grid. This is where signal is applied for amplification, and bias voltage is applied.
A tetrode has a second grid called the screen grid, between the control grid and the plate of the tube. It gets a positive bias (but no signal!) to cause more electrons to flow from the cathode to the plate, thus increasing current flow through the tube.
A pentode has a third grid, called the supressor grid, placed between the screen grid and the plate. When electrons hit the plate at high velocity they knock other elctrons loose from the metal, forming an electron cloud near the plate that can reduce current flow due to electron replulsion. The supressor grid is tied to the cathode and reflects these electrons back to the plate (they would normally be attracted to the positive charge on the screen grid). This improves efficiency.
If that's not enough there is another type of tube called a beam power electrode tube. Instead of a supressor grid it uses specially shaped metal electrodes to focus the electron stream onto the plate.
The 6L6/5881/KT66 is a beam power tube. The 6550/KT88, EL34 and EL84 are true pentodes.
As mentioned above you can use a tetrode, pentode. or beam power tube as a pseudo-triode by using only the control grid. If my memory is correct one normally ties the unused screen and suppressor grids to the plate.
A triode has one grid, called the control grid. This is where signal is applied for amplification, and bias voltage is applied.
A tetrode has a second grid called the screen grid, between the control grid and the plate of the tube. It gets a positive bias (but no signal!) to cause more electrons to flow from the cathode to the plate, thus increasing current flow through the tube.
A pentode has a third grid, called the supressor grid, placed between the screen grid and the plate. When electrons hit the plate at high velocity they knock other elctrons loose from the metal, forming an electron cloud near the plate that can reduce current flow due to electron replulsion. The supressor grid is tied to the cathode and reflects these electrons back to the plate (they would normally be attracted to the positive charge on the screen grid). This improves efficiency.
If that's not enough there is another type of tube called a beam power electrode tube. Instead of a supressor grid it uses specially shaped metal electrodes to focus the electron stream onto the plate.
The 6L6/5881/KT66 is a beam power tube. The 6550/KT88, EL34 and EL84 are true pentodes.
As mentioned above you can use a tetrode, pentode. or beam power tube as a pseudo-triode by using only the control grid. If my memory is correct one normally ties the unused screen and suppressor grids to the plate.