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]0.95
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New NC98
National
18
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SW.54 $5.00 $2.61 $49.95
NC-88 12.00 6.54 119.95
NC. 125 20.00 10.89 199.95
NC.183D 40.00 21.77 399.50
HR0.60 54.00 29.00 533.50
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Issis4sssssscss
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or Your Money Back at end of 10dayTrial
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Now for the first time, a crystal
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electrical bandspreod on amateur
or SWL bands, an RF
stage and 2 IF stages.
if/
Write, wire, phone or visit either store today.
Butler 1, Missouri
Bob Henry,
WOARA
Butler, Mo.
Ted Henry '
W6000
Los Angeles
i /./ Give.
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11240 West Olympic Blvd. Los Angeles es 64
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Phono Amplifier
(Continued front page 43)
former. One method of connecting the
transformer will give a lower volume
than the other; the proper connection
is the one with the lower volume.
After this correct phasing has been
established, then (3) return the plate
resistor, R,:,, to the plate of the 6V6
again.
Since the power supply must furnish
the negative grid biasing voltage for
the 6V6, the power supply design had
to proceed through a number of successive
modifications before it reached
its final form, as shown here in the
schematic diagram. Since the entire
amplifier is designed to operate at a
low volume level, this reduces the
plate supply current drain and permits
using the smallest possible power
transformer. Any one of the standard
makes of 250 -0-250 volt, 40 milliampere
replacement type power transformers
will satisfy the requirements. However,
since the amplifier requires 40
milliamperes, this leaves no factor of
safety in the design; in the interests
of safety the amplifier should be
operated in its enclosure for a number
of hours to check for excessive heating.
When using an output stage having
a considerable amount of feedback, as
is the case here with the two -loop circuit,
it is possible to keep down to a
very small level of output hum with
only a simple power supply filter. In
this application, the filter choke was
eliminated entirely, and was replaced
by a 500 -ohm, 2 -watt resistor, R,,. The
d.c. component of voltage across this
resistor is 20 volts, just the right biasing
value for the 6V6 output stage.
Therefore, this resistor was removed
from its usual position in the positive
lead and, instead, placed in the negative
lead, where the d.c. component of
voltage drop will be negative with
respect to ground. Then by using a
simple RC filter circuit, to eliminate
the a.c. components across the resistor,
the 6V6 biasing voltage is obtained
directly.
The 1800 aµfd. condenser, C,, at the
volume control, R,., is chosen to give
a drooping response at the low frequencies,
since the poor acoustic loading
presented by most record player
enclosures would not permit the extreme
low frequencies to be repro-
duced even if they were present. In
the case of adequate speaker loading,
the volume level of the amplifier is
sufficiently low that even if these frequencies
were to be reproduced they
would be below the threshold of audibility.
As an added consideration, if
the response of the speaker in a particular
enclosure should be of no value
below some particular frequency, trying
to force a bass response below
that frequency by brute force will
either overload the amplifier, or will
force the speaker into such excessive
motion that the speaker itself becomes
a non -linear element, and generates a
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