Improving Power Output

Author: N.H. Crowhurst

Let us start with the simplest tube circuit and see the various steps that are taken toward improving the available power output. Suppose we use a large version of the ordinary triode tube used for voltage amplification that will handle more power, using a plate coupling resistor and capacitor to get the power into the load.

Early audio output circuit.

We shall also have to use a high-impedance load suitable to the tube. (This was the way some early radio sets were designed.) Suppose that B+ is 250 volts and that we drop 120 volts in the load resistor. This means the plate potential will be 130 volts. (It will fluctuate, due to the audio, above and below 130 volts, and this fluctuation will be passed through the coupling capacitor to the load.) The audio fluctuation also appears across the plate coupling resistor, hence it will get not only the power necessary to feed the tube with plate current, but also some of the audio power due to the current fluctuations flowing through it.