Short intro to tubes:
- They're more-or-less low transconductance, low current density, depletion mode N channel FETs. No such thing as P-channel, which makes things difficult from time to time.
- Operating voltages are >10x higher than for most SS (solid state) circuits. So 100V supplies are kind of entry level, and 200-300V supplies are most common (with 500+ for power amplifiers and such; the largest ~MW size transmitter tubes are circa 30A and 30kV).
- Likewise, currents are lower, like 1-10mA for signal tubes, up to 100mA for outputs. The biggest common tubes are line output / sweep tubes, used for switching the deflection and flyback inductors in TVs; they go up to maybe 1.5A peak.
- Triodes are the only amplifying device that has a most nearly constant-voltage output characteristic. All others have the output characteristic (Id vs. Vds, Vg as parameter; Ic vs. Vce, Ib as parameter; Ia vs. Vak, Vgk as parameter) which rises sharply at low voltages, then levels off to a constant current ("linear") region.
- In a tetrode or pentode, the screen grid acts as the triode plate (while not drawing 100% of the current), in effect setting what the plate current will be.
You can also think of the screen as a second grid, where the current flow is due to the sum voltages on both grids. Difference being the screen is mu times less sensitive.
The last weirdness is, the cathode doesn't do jack until you heat it up.
So when I say heaters, I mean literal heaters, they make the cathode hot, so it can emit electrons! Archaic, right?!
The heater is insulated from the cathode, so it can only be so-and-so voltage away from it; it's possible they have two heater supplies for two sets of heaters, one around 0V and one around -200V or so.
Tim