T EVIS i - AmericanRadioHistory.Com

what 25,000 Servicemen
told Bill Anderson
(SYLVANIA Sales Service Engineer)
about PHOTOFACT...
SYLVANIA uir,rres CIS ,sr
-©.
W. J. ANDERSON
SYLVANIA Sales
Service Engineer,
Radio & TV Tube
Sales
"During the years of 1951 and 1952,
Robert Grow and have etalked to
approximately
located throughout the United
States. We have an excellent idea
ese
concerning the response of
men tor y PHOTOFACT
Service and
u
to your publications. Ih talking to
servicemen, I have heard many fine
compliments on the excellent job
your organization is doing. Such
comments as these are typical:
1. Very detailed and
t ás well as
instructions on any
pictures and schematics.
2. The theory of operation of vari-
fuel, such as found in the
PHOTOFACr
3. The immense amount of useful
short time presented c
fters release of the
manufacturer.
wave forms, as
4. w Pictures of measurements saves
time and increases profit to the
servicemen.
These and many more are typical
of the comments f serviceof
interest to you
men. This may
you continue
and your staff as y
to
lead the field in technical publica-
tions for the radio and television
servicemen."
ewd
Syi s,is Electric Products Inc.
Improved Power Supply
(Continued from page 42)
circuit. Plates go to opposite sides of
the high -voltage transformer winding.
Cathodes are connected together and
feed a regular condenser -input filter
CH,, C.
The grids of the rectifier tubes are
connected together through parasitic -
suppression resistors R,, R,, to the movable
arm of the potentiometer R,.
This potentiometer is the main voltage
control. A potential of around 400
volts is maintained across R:, by means
of an auxiliary positive supply consisting
of three small selenium rectifiers
Rect.,, Rect and Rect., and a single
filter condenser C:,. This auxiliary supply
is almost the only extra equipment
required as contrasted to an
ordinary condenser -input power supply
using a 5Y3 or 5U4 rectifier. Cost
of the extra parts is quite small and
the space required is negligible. A
6X4, 6X5, or similar rectifier tube can
be used in place of the selenium rectifiers
if desired, provided suitable filament
windings are available on the
power transformer.
The transformer used in this particular
power supply was a commercial
surplus unit having two 6.3 volt
filament windings, and a plate winding
of 700 volts center -tapped at 70
ma. There are few replacement -type
or new- equipment type transformers
on the market which have the necessary
two filament windings, although
some suitable types may appear with
the increasing use of the 6.3 volt
heater -type rectifier tubes such as the
6AX4. Hence, any handy transformer
delivering enough voltage will do, and
it may be necessary to add a filament
transformer to get the extra 6.3 volt
filament source that the circuit demands.
Output Characteristics
Fig. 3C shows curves of output voltage
vs output current. Maximum output
is 290 volts at 80 ma.; minimum
output is about 50 volts no load. The
minimum no -load voltage is essentially
the cut-off bias of the tubes at the
applied plate voltage. When the d.c.
output voltage is 50, the applied plate
voltage is 350 -50, or 300 r.m.s. When
the d.c. output voltage is high, say
300, the plate voltage is only about 50,
because the cathodes are so high
above ground.
The drop - off
in the regulation
curves at high voltage (300) is due to
the lack of adequate plate- cathode
voltage to give good current conduction
through the tubes. If more output
at high voltages is desired, use a
power transformer with higher secondary
voltage -400 or 450.
Limiting factor at low output voltages
is plate dissipation. Suppose we
want to draw 80 ma. at 50 volts. The
voltage across the tubes is 300, the
total current 80 ma.; power loss in
the tubes is 24 watts. This is a shade
over the rated dissipation of 10 watts
per tube --20 watts total. But at 250
volts and 80 ma., the actual plate
dissipation is only (350 - 250) X .08
= 8 watts, or 4 watts per tube. For
a generally beefier power supply, use
a transformer delivering about 450
volts each side, and a pair of 6L6's for
rectifiers. These bottles with the man -
sized bulbs are rated at 19 watts dissipation.
The small 6W6 has the virtue of
very low voltage drop at zero bias,
and so holds up better at the high end
than most other tubes, when the
available transformer voltage is limited.
However, 6V6's work nearly as
well -the droop in the top -voltage
output curve is a bit worse. 6K6's
and 6F6's will give about the same
results.
An outstanding virtue of this type of
power supply is its lack of fussiness
about tube types and other component
variations.
A 5" x 6" x 9" utility box was used
for the cabinet, the chassis a standard
unit cut down to fit inside. If tubes
larger than 6V6's are used, the box
will have to be larger. In any case,
cut plenty of ventilating holes in the
back.
An output voltmeter is very desira-
Fig. 4. Rear chassis view of the cathode- follower supply showing the parts layout.
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HOWARD W. SAMS & CO., INC.
2203 E. 46th St., Indianapolis 5, Ind.
114
HOWARD W. SAMS & CO., INC.
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