Valve Radio and Audio Repair Handbook

Chapter 7

What you need to know about valves (2)

High, medium and low impedance valves each have certain advantages over the others which make them suitable for specific jobs in a radio receiver. Taking them in order, high impedance types are best suited to what are called resistance-coupled amplifiers, used to build up weak voltages or signals. The circuit of a typical example, with batteries to supply the filament, the anode voltage and the grid bias, is shown in Figure 7.1. As mentioned in the previous chapter, the optimum value for the load resistor in the anode circuit of the triode is usually calculated to be about 2 or 3 times that of the r_a of the valve and is typically of between about 50 kΩ and 1 MΩ. Also, as mentioned earlier, the voltage ...