Hammond Organ L-100 Series, Руководство по обслуживанию

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Page 17
2–1 GENERAL
This section contains circuit descriptions of the amplifier chassis,
and the percussion attachments. There are three amplifier
assemblies in the L–100 series instruments. On the upper shelf
as viewed from the rear, to the left is the vibrato amplifier, towards
the center is the preamplifier and percussion amplifier, while the
reverberation and power amplifier is located on the lower shelf.
2–2 PRE-AMPLIFIERS
(See Figure 5–2 )
The preamplifier (V1) receives all signals impressed on the
matching transformer secondary, which originate by use of the
drawbars or preset tabs. Should any percussion tab be in use, a
portion of the second, third or both harmonics of the upper manual
will also appear in the input circuit of the per cussion amplifier
which will be discussed further on.
2–3 VIBRATO PHASE SHIFT AMPLIFIER
(See Figure 5–2 )
The vibrato system varies the frequency of the tones by
continuously shifting their phase. Circuit components include three
series -connected vacuum tube phase shifter stages (V2A, V2B,
and V3A), associated saturable reactors (SR101, SR102, SR103),
voltage amplifier (V3A), vibrato oscillator (V4A), and driver stages
(V4B, V5).
A single low frequency oscillator (V4) provides the rate for the
vibrato system (approx. 6.8 CPS). With either the normal or
small vibrato tab in use, this oscillator impresses its signal on
V4, a cathode fol lower and isolation stage. Positive pulses now
appear on the grid of driver tube V5. The plate circuit of this tube
is in series with three saturable reactors located in the plate and
cathode circuits of the phase shift stages. Irrespective of which
vibrato stop is used, the rate remains constant, but the degree of
vibrato is determined by the amplitude of the positive pulse on the
driver tube.
The continuous phase shift is accomplished by using 180° out-
of-phase signals from the plate and cathode of each shifter stage
and controlling them with the saturable reactors. Plate and cathode
resistors are of equal value and consequently signals are equal in
amplitude in each plate and cathode circuit. The saturable reactors
serve as a means of providing a varying composite of signals from
both plate and cathode of each stage, ranging from virtually full
cathode signal to full plate signal.
The driver tube plate current varies from about.5ma to 5ma at
vibrato rate. This current varies the degree of saturation in the
reactor cores and results in a smoothly varying impedance.
At minimum driver current (when the voltage feeding driver
tube V5 is negative and driver tube is nearly cut off) the reactor
impedances are maximum and are large compared to the 15,
000Ω plate circuit series resistors R104, R110, and R115.
SECTION II
THEORY OF OPERATION