how do pentodes work?

[Hal]

I got a book out of my school library, that hadn't been taken out since the early sisties....and i finally understand how tubes work!  Its really, really, simple :-D

At least i get diodes and triodes.  But how do pentodes work ?

[R.G.]

All tubes work by controlling the flow of electrodes forced off a cathode by either thermal energy or electric field concentration.  The thermal ones are the only ones that we need to think about.

The thermal emission makes a cloud of electrons around the cathode. A positive charge on the plate attracts the electrons to it, and current will flow from plate to cathode because the negatively charged  electrons are flowing from cathode to plate. Current will not flow the other way because there are no positively charged particles to carry the currrent. (OK, OK positrons exist, but as antimatter, they're incredibly rare here, and no tubes ever produced on earth use them.) We have a thermionic diode.

If we insert a grid between cathode and plate, we can choke off some of the electron flow by making the grid negative with respect to the cathode. That repells the electrons back to the cathode space charge (notice how I snuck in that technical term?) either partially or completely depending on how negative we make it.

The triode suffers from poor high frequency response because of the capacitance betweel plate and grid. If we put in a screen grid between the grid and plate, we can literally shield the plate from the grid by introducing an effectively grounded surface between the grid and plate. The screen grid also helps accelerate the electrons toward the plate, and as a result of its screening and evening the flow of electrons, the flow of electrons is much less affected by the plate-to-grid or -cathode voltage, so the plate characteristics of the tube show a flatter current vs grid voltage, less affected by plate voltage variations, in addition to high frequency response. We have a tetrode.

The tetrode is great, but with low plate voltages, the electrons flying into the plate are hitting hard enough to knock other electrons back off. This is secondary emission, and it gums up the works at voltages of about 100V and less. Inserting a coarse grid near the plate, but with the voltage of the cathode forces any secondary emission electrons back into the plate. This removes the secondary emission kink in the plate characteristics at low plate voltage, and we now have a pentode.

Interestingly enough, in the UK, the beam-power form of the pentode is called a "kinkless tetrode" and this is what accounts for the "KT88" and other KTxx numbered Brit tubes.

So - a pentode works just like a triode, but with better high frequency, better efficiency, and as a side effect, higher gain. The grid still shuts off the flow. The screen and suppressor grids just make sure that the electrons follow the rules in an orderly way - like grade school kids flowing more smoothly under the watchful eye of a crossing guard.