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Point B--Plate of R.F. Tube
Back    At B, the plate of the r.f. tube, the signal should be heard with increased strength. Normal gain is about 25 times but may be more or less dependent upon bias voltage.

   It is worthwhile here to inquire what we expect of a vacuum tube in this part of the circuit.

   In a receiver the vacuum tube may be used to perform a number of highly important function, namely:
  1. Amplification of a.c. energy without appreciable distortion of waveform.
  2. Rectification of a.c. energy to d.c. energy.
  3. Generation of a.c. waves of high frequency.
  4. Demodulation of a modulated wave train; i.e., recovering the intelligence impressed upon a "carrier".
  5. Frequency changing.
   In Fig. 4 the first tube is a radio-frequency amplifier. Its function is to amplify the r.f. voltage selected by the preceding resonant circuit.

   To appreciate what is happening in the tube circuit, it is important to have a clear understanding of the following fundamentals:
  1. If a tube is working normally, a reasonably exact reproduction of the voltage applied to the input grid will be found across the plate impedance.
  2. The output voltage may be greater or less than the input voltage depending upon the mutual conductance of the tube and the magnitude of the impedance offered by the plate load at the impressed frequency.
   The mutual conductance depends upon the grid bias. In Fig. 4 this is the a.v.c. voltage.

   When a resistor is used as a plate load, that part of the total plate impedance may be considered constant at all frequencies. The function of a tuned plate impedance usually a parallel resonant circuit, is to present a much higher impedance at the desired frequency than at any other frequency. In this manner the selected signal is amplified much more than any other signals that may be present at the input.

   All this means that if an amplifier tube is working properly, a signal voltage impressed upon the contrl grid will reappear upon the plate.

Trouble
   Failure of the signal to appear at B indicates that the tube is not working. This may be caused by:
  1. Defective Tube.
  2. Wrong voltages or absence of plate voltage.
   In shooting trouble, one of the principal tests is a voltage check at the tube socket. 		Next
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