[Ktechlab-devel] Elements inside a 2A3

[Alan Grimes]

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.

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