In searching for the best method of emulating a triode power tube using a inexpensive (widely percieved to be inferior) pentode or tetrode I found myself trying to think of a good answer to a question that occured to me. Why do you need to connect the screen and suppressor grids to anything. Why not just connect the tube as a triode using only the signal grid, the plate and the cathode. I know that there is a very practical and logical reason for not doing it but it is eluding me. Anyone in the mood to answer a basic tube theory question and help me reduce my acute case of ignorance and bafflement to a merely moderate one? 
If you leave electrodes disconnected, they will gently charge up to a potential simply because of the capacitance between themselves and nearby (connected) electrodes. The thing is, that potential may not be the potential you want, so you define it by connecting to electrodes of your choice, thus avoiding undefined potentials from adversely affecting the valve's operation.
I regulary do this in my amp driver stages....in push pull with a common cathode resistor......but watchout with resistor value doesn't exceed the screen grid current. The g2 will only handle a few mA. It's a damned good way to get a linearized stage......from unmatched tubes.
<pentode gain is too high and unpredictable and decent triodes too low>.
Okay one can use anode to g1 feedback to tame a penny but then one lowers the input Z and reduces thd......... reducing input Z isn't aways my desire as the previous stage then has greater loading and increases the thd.
Anything else ?
rich
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