Hammond Model A, Руководство пользователя, Страница: 129 / 298

THE HAMMOND ORGAN WITH PERCUSSION
Percussion tones are available only on the upper manual (with the B adjust key
depressed) of all consoles with the suffix -3 in their model designation. These
consoles, except for the four percussion control tablets in the upper right hand
corner, look and function similar to consoles with the suffix -2 in their model
designation, when the percussion effect is not in use.
1. THEORY OF OPERATION
The percussion tones are produced by borrowing the 2nd or 3rd harmonic signal
from the corresponding drawbar (of the upper manual B adjust key drawbar group),
amplifying it, returning part of it to same drawbar, and conducting the balance
through push-pull control tubes, which when keyed cause the signal to fade away at a
pre-determined rate.
2. GENERAL CICUIT OPERATION (All Reference Is To Figure 24 Section 16, Part 1)
With percussion tablet on, upper manual B adjust key and an upper manual playing
key pressed, the 2nd or 3rd harmonic signal appearing on an upper manual busbar is
conducted through B adjust key drawbar wire to input of percussion amplifier
(terminal H) and amplified by T4 and V5. Besides providing push-pull signal for the
control tube V7, the percussion input transformer T5 has a third winding which feeds
the signal back to the 2nd or 3rd harmonic drawbar through equivalent key circuit
resistor R50 and terminal J. Thus the signal that was borrowed from the 2nd or 3rd
harmonic drawbar for the percussion amplifier is replaced.
When a key is depressed the signal first sounds loudly through the control tube,
transformer T6, a high pass filter, and terminal D to the grid of V4. Immediately
condenser C31 in the control tube grid circuit begins to discharge, causing the
signal to fade away. Terminal K (approxi25 volts) is connected to the
8th harmonic B adjust key drawbar wire which is connected to manual busbar. When an
upper manual key is pressed, terminal K is grounded through the tone generator
filters. This virtually grounds the plate of V6 (connected as a diode), stops
conduction, and isolates cathode and control tube grid circuit. The grid then drifts
from approxi25 volts to about +15 volts, at a rate determined by the time
required for C31 to discharge through R57 and R58. At the completion of this
sequence the percussion signal is blocked. No further percussion effects occur until
all keys of the upper manual are released and control grids can again rise to +25
volts. The rate of this rise is fixed by the time required to charge C31 to +25
volts through R55 and R56.
3. FOUR PERCUSSION CONTROL TABLETS, CUTOFF CONTROL, AND THEIR FUNCTIONS
The Percussion On-Off Tablet when turned on does five things to the signals of the
upper manual B adjust key drawbars.
(a) It disconnects the 2nd harmonic drawbar from its signal wire.
(b) It disconnects the 3rd harmonic drawbar from its signal wire.
(c) It connects the 2nd or 3rd harmonic drawbar signal wire (depending on position
of Harmonic Selector Tablet) to input of percussion amplifier.
(d) It disconnects the 8th harmonic drawbar from its signal wire. This wire
(connected through generator filters to ground when any key is pressed) is connected
to terminal K. The 8th harmonic signal is not available on the upper
manual as long
as percussion tablet is on.
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