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

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A number of methods can be used to obtain a negative voltage on the grid of RF devices; two
different methods are used in the FM section of this tuner. Tubes V51, V53, V54, and V56
employ cathode bias, where the current drawn by the tube is passed through a resistor in the
cathode circuit, causing the cathode to become positive with respect to ground. Because the grid
is tied to ground through a resistor or IF transformer, the cathode will be positive with respect to
the grid, which is the same as making the grid negative with respect to the cathode.
Contact bias is used for V52. If the cathode of a tube is tied directly to circuit ground and the
grid returned to ground through a high resistance, a very small amount of current will be drawn by
the grid. This current will be limited by the resistor however, and a slight negative voltage will
appear at the grid. Biasing in this manner is useful where cathode impedance must be kept low
and the signal level is low.
AGC is obtained by feeding the negative dc voltage developed at the V55 ratio detector and
supplying it to V51 and V53 (the RF and first IF amplifiers) through an isolating network
consisting of R61 (330 k
Ω
) and C53 (0.01
μ
F). This network filters out all RF, IF, and audio
energy, preventing interaction between the input and output stages of the tuner. The dc voltage
developed at the detector output is proportional to the incoming signal strength and so is useful as
a gain-controlling voltage. When the bias at the grid of a tube is increased in a negative direction,
the gain will be reduced. Thus, the stronger the signal, the less sensitive the tuner becomes, which
tends to keep the audio level relatively constant and prevents overloading when tuned to very
strong signals.
1.2.1 Multiplex Section
3
The FM tuner recovers the audio and multiplex information from the FM carrier and presents this
signal to the multiplex decoder consisting of V57, V58, V59, and associated components. The
signal contains the L + R and L – R information, plus the 19 kHz pilot tone.
Tube V57a amplifies the signal to compensate for any losses and V57b lowers the impedance
of the signal, operating as a cathode follower. Coil L56 and capacitor C83 act as a low-pass filter,
which passes only the L + R signal. This signal can then be adjusted to the proper level by
Separation Control R86 for insertion into the matrix network.
3. The description of the multiplex circuit and portions of Figure 1.2 are adapted from: “Assembly and Operation of the Heathkit
Multiplex Adapter Model AC-11,” the Heath Company, Benton Harbor, MI, 1961.