WhitakerAudio AM/FM Stereo Tuner 2012, Сборка и руководство пользователя

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Note that the 6BE6 (V2) has five grids, a plate, and a cathode from which the name heptode
converter is derived. The first and second grids along with the cathode comprise a triode, which is
employed as a Hartley oscillator. A tuned circuit is connected between the first grid and ground,
with the cathode connected to a tap on the coil near the ground end, thus providing feedback to
maintain oscillation. The B+ voltage is connected to the #2 and #4 grids, which are tied together
internally within the tube. The #2 grid becomes a plate for the triode section and the #4 grid a
screen grid for the pentode section, containing the cathode, the #3 control grid, the #5 suppressor
grid, and the anode (plate).
Signals from the V1 RF stage are amplified again at the V2 6BE6 mixer stage, along with
another signal coming from the oscillator contained in this same stage. 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 mixer, as well as the RF and oscillator signals. The
oscillator frequency is selected so it is always 455 kHz higher than the tuned frequency of the RF
section. Therefore, the difference will always be 455 kHz. It is to this frequency that the
intermediate frequency (IF) transformers are tuned. This function of changing frequencies is the
well-known as the Superheterodyne Principle. Improved selectivity and gain is obtained due to
the fixed-tuned IF transformers, which are designed to provide optimum performance at one
frequency only.
RF energy from the plate of the mixer is coupled through the first IF transformer (T3) to the
grid of the 6BA6 IF amplifier (V3), where the 455 kHz difference (intermediate frequency) is
amplified. The first IF transformer will pass the 455 kHz signal and reject almost all others. The
amplified IF signal is taken from the plate of the 6BA6 (V3) to the second IF transformer (T4),
where residual signals are further reduced.
Amplitude modulation consists of a carrier frequency that is heterodyned or mixed with the
audio modulating frequency at the transmitter, producing four frequencies of which three are
transmitted. A similar function takes place in the V2 6BE6 mixer in that the carrier frequency will
be present along with the sum and difference of the carrier and audio, called sidebands . The audio
signal, per se, is not transmitted. Rather, the sideband distance on each side of the carrier
frequency is determined by the audio frequency—higher modulating frequencies spreading the
sidebands farther apart. Sideband power is determined by the percentage of modulation, with 50
percent of additional power present in the sidebands at 100% modulation, the fundamental power