...afraid to ask!

[old_tv_nut]

Originally Posted by Chris K

OK...since I'm a bioscience guy who needs explanations at the amino acid level, I'm trying to reduce what's going on with the grid to a physical state. Biased at 0%, a tube grid is a grid or screen of fine wires with no voltage, and hence, no free electrons present on it. A tube biased at 100% has a screen saturated with the maximum number of electrons present it can retain. I think I understand the cathode to plate flow part that's regulated by biasing of the grid but now, I'm looking to better understand the grid itself and what's going on with it when the tube is biased.

In most cases, the control grid is biased negative with respect to the cathode. "Free electrons" is a poor decriptor of what's going on. All metals have "free" electrons. A better term is a negative charge consisting of excess electrons.
Also, you would say "the tube is biased" at a certain current, not "the grid is biased at a certain percent."

In the case of a linear "class A" amplifier, the maximum possible current (which you called 100%) is determined by the power supply voltage and load resistor when the voltage across the tube (plate with respect to cathode) is zero. Then, the entire power supply voltage is applied to the load resistor with zero volts appearing across the tube. This may be a hypothetical case, because it may be impossible for the tube to draw that much current.

In general, a class A amplifier stage will be biased at a point where the quiescent bias current is halfway between zero and the maximum possible, so there is an equal range above and below the bias point for the output signal to vary up and down.