Hammond Organ L-100 Series, Service Manual

Page: 18 / 87

Page 18
Therefore, under this condition most signal will emanate from
the plate. (The reactors being virtually short circuited by the
plate circuit series resistors) and phase shift will be maximum —
approaching 180°—since plate voltage is 180° out of phase with
grid voltage.
At maximum driver current (when voltage feeding driver tube V5
is positive and driver tube is conducting maximum current) the
reactors are saturated and their impedance is a minimum—small
compared to the 15, 000Ω plate circuit series resistors R104, R110,
R115. Therefore, most signal will emanate from the cathode (the
saturated and low impedance reactors virtually short circuit the
plate circuit series resistors) and phase shift will be a minimum—
approach ing 0°—since cathode voltage is in phase with input grid
voltage.
Between these extremes, the phase varies smoothly under control
of the saturable reactors.
The continuous change in phase is equivalent to a continuous
frequency variation, and thus the frequency varies up and down at
vibrato rate.
2–4 PERCUSSION AMPLIFIER
(See Figure 5–2 )
With either or both of the 2ND or 3RD HARMONIC tabs
depressed, the signal will be impressed upon the input of the
2N306 transistor. The output of this transistor is resistance coupled
to the one half of V11 which acts as a control tube and is normally
conducting, so when a key is depressed the percussive note first
sounds loudly. It passes through the control tube and a band pass
filter and is impressed on the grid terminal of V1.
Immediately the note begins to fade away, giving the characteristic
percussion effect. This fading is accomplished as follows: When
either harmonic stop is depressed the keying wire (normally held at
+28VDC through anti-spark resistor R215) is connected to the solo
manual 6th harmonic drawbar. When a key is pressed this keying
line is grounded through the key contact and tone generator filter.
This virtually grounds the grid and plate or V11 (connected as a
diode) open-circuiting the tube and isolating the control tube grid
circuit. The grid of the control tube drifts from its operating potential
of about 25 volts to a cutoff potential (about plus 15 volts) at a rate
determined by the time required for C210 to discharge through
R219 and R409.
The percussion signal is now blocked. No percussion notes can
sound until all keys of the solo manual are released and the control
grid again rises to plus 25 volts. The time of this rise (that is, how
quickly the control tubes turn on again after the key is released) is
the time required to charge C210 to plus 25 volts through R218
When a “percussion” tab is pressed, the solo manual second,
third, or both harmonic manual busbars are connected to the
green percussion signal line and a 5Ω series resistor is connected
between the manual bus wire and drawbars providing for a sus-
tained signal in addition to the percussion signal. The 6th harmonic
drawbar is dis connected from its lead wire and this wire (which is
grounded through the generator magnets when any key is pressed)
is used to turn off the control tube. Therefore the 6th harmonic is
not available on the upper manual when the percussion is in use.
When the PERCUSSlON SOFT tab is down, it reduces the volume
by shunting resistor R224 into the dividing network composed of
R222 and R223.
The PERCUSSION FAST DECAY tab determines how fast the
sound fades away after a key is pressed. When the tab is up,
resistor R2l9 discharges capacitor C210, re ducing the DC voltage
on the control tube grids to cut-off in about 2½ seconds.
When the tab is down, resistor R409 is shunted across resistor
R219, reducing the time to discharge capacitor C210 and there by
reducing the DC voltage on the control tube grid to cut off in less
than one-half second.