Plate Detectors

Author: J.B. Hoag

Fig. 17 E. A simple plate detector

Figure 17 E shows a triode tube used as a detector. It is heavily biased onto the lower knee of its characteristic curve, near the cutoff, in order that it may operate as a rectifier. This is accomplished by the voltage drop set up by the plate current in R₁ (10,000 to 20,000 ohms). C₁ (0.5 μfd. or larger) bypasses both r.f. and a.f. around R₁. The incoming modulated carrier wave fluctuates the grid about the bias point and causes the plate current to flow in the manner shown in Fig. 17 F.

Fig. 17 F. Principle of plate detection

The r.f. component of this current is bypassed through condenser C₂ (0.001 to 0.002 μfd.). The average a.f. variation, indicated by the dotted line of Fig. 17 F, which duplicates the modulations of the input wave, sets up a corresponding a.f. voltage across the load resistor R₂ (50,000 to 100,000 ohms). This is transmitted to the output device through the coupling condenser C₃ (0.1 μfd.). Because small voltages on the grid can cause comparatively large voltage fluctuations across the load resistor, this circuit not only detects but also amplifies the incoming signal.

Fig. 17 G. A pentode plate detector

In the pentode plate detector of Fig. 17 G, the principle is the same as that for the triode. The bypass condenser C₄ of the screen must have a low reactance at both a.f. and r.f.; 0.5 μfd. or more is used. The resistors R₃ (50,000 ohms) and R₄ (20,000 ohms) are used as a voltage divider across the B supply to apply the proper potential (30 volts or so) to the screen. C₂ should be about 250 to 500 pF.; R₂ about 100,000 to 250,000 ohms; the other constants as for the triode.