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gmoon3 years ago
Quick explanation of a "common cathode" triode gain stage" (see the picture).

A simplified triode has a cathode, a grid and a plate. Electrons jump off the cathode and head to the plate (moving negative to positive), and the grid is the "valve." In a "cathode biased" gain stage (most amplifiers), the grid should be negative compared to the cathode, or too much current will flow all the time. The cathode bias resistor raises the voltage of the cathode, rather than dropping the grid voltage (it does the same thing).

Tubes are "voltage in, current out" devices. Voltage gain on the grid makes current gain on the plate. In order to convert current gain into voltage gain (so it's voltage in, voltage out), we place a load resistor on the plate. If we vary the current on a fixed resistor, the voltage HAS to change, too--Ohms Law tells us that. So the load resistor converts current change to voltage change. The higher the load resistor, the greater the voltage gain. There's a trade off, of course--too big a load resistor and there isn't enough current to do the job and the output impedance suffers.

This trick with a load resistor is also how common-emitter transistor amplifier circuits work, too.

The components in the red circles are just capacitive coupling RC pairs. It's important to block any DC offset that results from the bias voltages, and a capacitor does that. Generally when you have a capacitor, you need to follow it with a resistor to form a low-pass RC filter--because the filter effect is going to happen anyways (with the impedance of the subsequent components) and adding a resistor let's you control the roll off explicitly.

For this simple drawing, I've omitted the heater wires. They don't carry any signal and wiring them is simple, so sometimes they are left out (you'll see this on schematics all the time, particularly for opamps.)
triode basic1.jpg
gmoon gmoon3 years ago
One more note about plate load resistors--as they get larger, the plate voltage drops, too. So going larger only adds gain to a point...
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