Tetrode has two grids and
Penthode has three grids
If all grids are shorted than the bulb works the same as triode. If not than each grid can be used as an extra gain stage to amplify the voltage.
If grids have different polarity but the same offset DC voltage, they create a unity gain buffer i.e negative feedback.
Penthode has three grids
If all grids are shorted than the bulb works the same as triode. If not than each grid can be used as an extra gain stage to amplify the voltage.
If grids have different polarity but the same offset DC voltage, they create a unity gain buffer i.e negative feedback.