WhitakerAudio AM/FM Stereo Tuner 2012, Assembly & User Manual

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isolating the grid from the cathode via R54 (470 k
Ω
). Thus, the grid remains at a fixed potential
while the cathode voltage is varied, causing the tube to act as though the grid potential were
changing. This operation is much the same as a grounded-grid amplifier. Loading of V51b is
provided by R55 (10 k
Ω
), which is tied to B+, and this load is tuned through C61 (3.3 pF) to L53
on the tuning capacitor assembly. The main advantage of this circuit is that high gain, equivalent
to that of a pentode, can be obtained at a much lower noise figure.
The signal from the V51 RF amplifier is coupled to the V52 6U8 pentode grid through C62
(47 pF) and amplified. The triode section of V52 is used as an oscillator of the standard Hartley
type. Coupling of the oscillator signal to the mixer circuit is accomplished by stray capacitance
between C58 (47 pF) and C62 (47 pF). During assembly, C58 and C62 are placed close to each
other in order to accomplish this coupling. Since the oscillator and RF signals are both present in
the tube, they mix in such a manner that the sum and difference of the two frequencies are present
at the output of the pentode, as well as the RF and oscillator signals.
The oscillator frequency is selected so it is always 10.7 MHz higher than the frequency of the
RF section. Therefore, the difference will always be 10.7 MHz. It is to this frequency that the
intermediate frequency (IF) transformers are tuned.
Amplification of the IF signal takes place in the first 6CB6 stage, V53. The first IF
transformer (T53) passes the 10.7 MHz signal and rejects almost all unwanted signals. This signal
is connected to the grid of V54, the second IF amplifier stage. The signal is amplified by the tube
and fed to the second IF transformer (T54). Any residual unwanted signal that might remain is
eliminated by this transformer.
Detection of an FM signal involves a different principle than that used for AM demodulation,
due to the different nature of the transmitted signal. For FM, the carrier amplitude is held constant
and the carrier frequency varies on both sides of the center frequency at a rate determined by the
modulating frequency, and a frequency swing proportional to the volume of the modulating
sound.
It it is apparent that any amplitude variations on an FM signal contribute nothing to the
detected audio and so amplitude variations can be clipped off in the IF stages or canceled-out in
the detector. Random noise, ignition pulses from gasoline engines and electric motors, and static
from electrical storms are all forms of amplitude modulation that can adversely affect the