Lets review pins and elements using my favorite part, the 2A3 The 2A3 is a filamentary triode with 3 electrodes and 4 pins.
The pins are: Anode (a) Grid (g) and the filament (f1, f2) Note: g means Conductance A first approximation of the filament has a 1-ohm resistor between f1 and f2. (powered by a 2.5v supply) Several problems immediately present themselves. First, this model ignores the temperature coefficient of the filament. Second, there's the problem of computing the critical working voltage of the tube. ( V_a - avg(V_f1, V_f2) ) And third, there is the parasitic capacitance. All voltages in the analysis of a vacuum tube are referenced to the cathode. There are two ways to approach the second problem. The first is to split the resistor in half and add a node to the circuit, then compute all the triode properties off of that node. The second is to write a generalized triode equation that can handle a 2-node cathode (filament in this case) and perform the necessary averaging... So a model for f1 and f2 would have one resistor for each filament node, connected to an internal hidden pin which would also have two capacitors (the parasitic capacitance between the filament and the grid, and the filament and the anode respectively). The next set of internal elements are a pair of thermionic diodes (I don't know how these work just yet), The obvious one between the filament and the anode, and the second between the filament and the grid. Both of these diodes have a transfer function based on the voltage between said electrode and the filament. Finally, these diodes interact with each other, the grid will affect the transfer function to the anode at any practical voltage, and the anode will affect the transfer function to the grid when it is conducting. This is called transconductance. The 2A3 has a relatively small transconductance (about 4), so it can require a full-scale swing of as much as 95 volts between V_p (pinch-off voltage), and positive grid voltage (where the tube enters class A2 operation and the grid begins to conduct). My point is to show an interesting example where the number of elements dwarfs the number of pins. Attached is an attempt to simulate a 2A3 based on an elemental analysis. -- DO NOT USE OBAMACARE. DO NOT BUY OBAMACARE. Powers are not rights.
2A3equiv.circuit
Description: application/circuit
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