Power Amplifier - Hammond A Manual

5-52. No further percussion signal can be heard until all keys of the upper manual
are released so that the control tube V7 grids can again drop to -25 volts (the rate
of this drop is fixed by the time required to charge C31 to -15 volts through R55 and
R56). Thus the percussion effect is heard only when keys are played in a detached
manner; that is, when all keys are released before pressing the next one.
5-53. REVERBERATION UNIT. (See figure 3 in the Reverberation section) This device
simulates musically desirable echoes in a large room. An electrical signal
from the amplifier is applied to the driver coil in the reverberation unit, which
converts the electrical signal into a twisting movement of 3 coil springs. This
motion is transmitted along each spring to a pickup unit, where part of it is
converted back to electrical energy. The remaining portion is reflected back
to the driver and again back to the pickup after a time interval determined by the
spring length. This reflection process continues until the signal level is reduced to
about one millionth of its signal value so that it is no longer audible. The springs
are different in length and thus there are 3 separate sets of echoes, each repeated a
number of times. Electronic amplification circuitry associated with the reverberation
unit is contained in the power amplifier, described below.
5-54. POWER AMPLIFIER. (See figure 26 in Tone Cabinet Amp section of Wiring and
Schematic Diagrams )
5-55. This is a 3-channel amplifier with 2 treble channels (one for non-reverberated
and one for reverberated signal) and a bass channel, with a cross-over point of 200
cycles. Each channel has two 6BQ5 output tubes with self bias. Each treble channel
drives a 12" speaker, and the bass channel drives two 15" speakers in parallel.
5-56. The power supply unit is a separate chassis housing the power transformer,
rectifier tubes, filter, and input connections for power and signal. A 6-pin plug
engages the console cable, and a 5-pin receptacle is provided for plugging in
additional tone cabinets. The console cable consists of 5 conductors; 2 for AC power,
2 for push-pull signal, and ground.
5-57. The push-pull signal from the console (G1 and G2) drives treble input tube V1.
Resistance capacitance filters ahead of V1 filter out signal frequencies below 200
cycles. V1 drives output tubes V2 and V3 of the treble direct channel. It also drives
double triode tube V9 which, in turn, drives the reverberation unit.
5-58. The output of the reverberation unit passes through transistor TR-1, and part
of the signal goes to the treble reverberation switch. This adjusts the amount of
reverberated signal going into V1O, which drives output tubes V11 and V12 of the
treble reverberation channel. The switch, in its off position, picks up signal from
input terminal G1, in order to make use of the channel for non-reverberated signal
when the treble reverberation is off.
5-59. Both treble channel output transformers have tertiary windings which supply
inverse feedback signal to the cathodes of the output tubes.
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