Hammond L series User Manual

SECTION
II
THEORY
OF
OPERATION
2-1.
GENERAL.
-
This
section
contains
circuit
descriptions
of
the
ampUfier
chassis,
and
the
percussion
attachments.
There
are
three
amphfier
assembhes
in
the
L— 100
series
instruments.
On
the
upper
shelf as
viewed
from the
rear, to
the
left is
the
vibrato
amphfier,
towards
the
center
the
preamplifier
and
percussion
amphfier,
while
the
reverberation
and
power
amplifier
is
located
on
the
lower
shelf.
2-2.
PREAMPLIFIERS
(See
Figure
5-1).
-
The
pre-
amplifier
(VI)
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
wiU
also
appear in
the
input circuit
of the
per-
cussion
amplifier
which
will be
discussed
further on.
2-3.
VIBRATO
PHASE SHIFT
AMPLIFIER
(See
Figures
5-1
through
5-4).
-
The
vibrator
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
(SRIOI,
SR102,
SR103),
voltage
amphfier
(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
controUing
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
15000
ohm
plate,
circuit
series
resistors
Rl
04,
Rl
1
0,
Rl 1 5.
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
1
80°
-
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
1 5OO0
ohm
plate
circuit series
resistors
R104,
Rl
10,R1
15.
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
equi-

Hammond L series Specifications

General

Brand	Hammond
Model	L series
Category	Musical Instrument
Language	English