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The Stereo 35 may also be used with an FM tuner
without need for a preamplifier, provided that the tuner
is equipped with a volume control and switched a.c. outlet,
and has an out put of at least 1 volt.
The loud speakers chosen may be of any impedance or
efficiency, since the Stereo 35 has sufficient power reserve
beyond that usually required for home listening so that no
lack of volume is likely to be observed, even with relatively
inefficient speakers. The loudspeakers connection for each
channel is made between the COM (common) and either
the 8- o 16-ohm tap provided; the bottom plate is printed
to identify these terminals. The 8-ohm tap will permit
satisfactory operation with loudspeakers of 4- to 12-ohm
nominal impedance, while the 16-ohm tap may be used with
loudspeakers of 12- to 20-ohm rating.
PHASING THE LOUDSPEAKERS
The two loudspeakers used in a stereo system must be
phased properly for correct stereo reproduction. The phas-
ing process itself is simple, after the amplifier is operating
and the two loudspeakers are connected.
Play a monophonic record, or tune in a monophonic
broadcast on your tuner. As you walk across the room in
front of the loudspeakers, there should be a smooth, con-
tinuous transition of sound from one side of the room to the
other. When you are directly between the loudspeakers,
the sound should appear to come from a source that is
directly in front of you, that is, also centered between the
loudspeakers. If you have any doubt as to whether or not
the phasing is correct, after making this test, transpose the
two connections at one of the amplifier terminal strips; that
is, connect the wire which was connected to "8" or "16" to
"COM", and vice versa. If you have changed to the correct
phasing, the sound will now be distinctly centered, and the
bass tones will appear to have increased. If you have
changed the connection to incorrect phasing, the sound
will seem to jump from one side to the other as you move
slightly to the right and left of center.
LISTENING WITH STEREO HEADPHONES
Headphones can be used with the Stereo 35 in place of
loudspeakers or connected in addition to loudspeakers.
Most headphone manufacturers supply instructions and
special junction boxes which permit direct connection of
headphones to the loudspeaker outputs of the amplifier.
With low impedance headphones (4 to 16 ohm rating),
without instructions from the manufacturer, a 1-watt resistor
of about 100 ohms should be connected in series with the
output of each channel. This will attenuate the signal to each
headphone, improving the signal to noise ratio.
CONNECTING A THIRD LOUDSPEAKER
TO YOUR STEREO SYSTEM
In most stereophonic systems, the usual arrangement of
two loudspeakers which form an equilateral triangle with
the listener's position will yield optimum performance. In
cases where the program contains exaggerated separation,
judicious use of the blend control on the preamplifier will
provide a more natural and pleasing effect.
When room arrangement or size does not permit the ideal
placement of loudspeakers, the addition of a third loud-
speaker, playing a combination of the left- and right-
channel material, may improve the spatial effect consider-
ably. Those listeners who feel a need for a third loudspeaker,
can try this arrangement without need for a third amplifier
or a special output on their preamplifier by using the follow-
ing method. A monophonic extension loudspeaker may be
connected to a stereo system in the same way.
The circuit shown below provides output from a central
loudspeaker with full control over its level. For best results,
the speakers used should be identical.
Be certain that the loudspeakers are properly phased.
IN CASE OF TROUBLE
The Stereo 35 has been carefully engineered to give
long, trouble-free service. However, a mistake in assembly
or a defective component can affect its performance or
make it inoperative. If difficulty is encountered, a method-
ical analysis of the trouble will usually lead to a quick
cure. The first thing to check is the wiring to be sure that
all of the steps have been performed, and that all connections
have been soldered. It is especially helpful to have someone
other than the builder do the checking.
If the tubes do not light at all ( on both channels), it is
likely that a.c. power is not being delivered to the amplifier.
In all likelihood, the fuse will have blown, indicating the
possibility of a short-circuit in either the filament or high
voltage supply lines. If the resistance between chassis
ground and the quad filter capacitor lugs is less than
100,000 ohm, this indicates a probable short-circuit in
the high-voltage supply.
If the tubes of one channel light, but those of the other
do not, the filament leads from the power transformer to
one of the circuit boards are probably non properly con-
nected, or these may be defective in the power transformer.
If all tubes light but one, the tube in question should be
interchanged with one of the same type from another socket
on either board. This should show whether the tube or
its associated circuitry is at fault; if the tube still fails to
light, it is faulty. If it lights in its new position, and the
tube inserted in its place is now dark, a close examination
of the tube socket wiring will usually reveal the source of
the difficulty. Should an output tube (6BQ5) ever require
replacement, it is preferable, but not essential, that the
pair be replaced.
If all of the tubes light, but there is no sound from the
loudspeakers when the system is in operation, it is first
necessary to eliminate other components in the system as
possible cause of the trouble. Inserting or removing the
input plugs at the input sockets of the Stereo 35, while it
is on should produce a loud momentary hum or "thump".
If the loudspeakers are properly connected, and no noise
is produced by inserting or removing the input plugs, the
high voltage supply is probably not connected. check the
wiring around the quad filter capacitor. If one channel
plays, and the other does not, check the signal path in the
defective channel, as well as the high voltage supply. An-
other possibility is that strands of wire from the output
transformer leads going to the output terminal strip are
accidentally shorting, or that the wires leading to the loud-
speakers are touching the chassis or each other.
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