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From: mcovingt@athena.cs.uga.edu (Michael A. Covington)
Newsgroups: sci.electronics
Subject: Re: Question about Vacuum Tubes.
Keywords: Tubes
Message-ID: <1990Dec27.191949.9562@athena.cs.uga.edu>
Date: 27 Dec 90 19:19:49 GMT
References: <1990Dec27.020813.4864@yenta.alb.nm.us>
Distribution: usa
Organization: University of Georgia, Athens
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OK, here are some tube basics.

Tubes act like FETs, with cathode=source, control grid=gate, and
plate (anode) = drain. That means that with a triode tube, you have
those three pins, plus two pins for the heater.

Two additional grids are sometimes used. In a pentode tube the additional
grids are called the screen grid (usually connected to a constant positive
voltage close to that of the plate supply) and the suppressor grid
(grounded). These introduce shielding between the control grid and the plate.
Because the plate is so large, the capacitance from control grid to plate
would be large if there were no shielding in there.

Also, a pentode has a characteristic curve like a FET, i.e., it is basically
a current source controlled by voltage. A triode's characteristic curve
consists of slanting rather than horizontal lines, like an FET at low voltage.

Of course many popular tubes have more than one tube in the same envelope.
For instance the 12AX7 is two triodes.
.