T EVIS i - AmericanRadioHistory.Com

IMPROVED VARIABLE
POWER SUPPLY
By
WILLIAM CREVISTON
An adaptation of an English circuit which provides outputs
Fig. 1. Front view of the power supply unit.
It is housed in a 5" x 6" x 9" utility box.
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PLI
6.1v rt.
R. ,-I0.000 ..h n. w. res.
R,- 250.000 ohm rol. control, (mear-taper
C, -40 pjd.. 450 r. eles. cond.
C,-20 p¡d., 450 v. elec. cond.
pjd.. 450 v. elec. Bond.
CH, -I0 hy., 80 ma. filter choke
.11, -0.400 volt d.c. meter
Rea. t, Rees. c, Rect. r 75 ma., 130 r. selenium
rectifier
v. pilot lamp
F, 117 y. line
S,
¡use
S.p.s.t. toggle switch
T1-Power trans. (see tex))
V V,-611.6 nube (.e,' text)
Fig. 2. Circuit of variable power supply.
Output is adjustable from 50 to 300 volts,
70 ma. Regulation is same as regular condenser
-input pack at all voltage settings.
from 50 to 300 volts using a cathode -follower type hookup.
THE most basic necessity in experimental
work with vacuum tubes is
a power supply, preferably one
which will deliver a wide range of
different voltages. A small box with
a knob on the front, which will deliver,
say, 50 to 300 volts d.c., plus some
filament power, will serve not only
for powering experimental tube circuits
but in calibrating meters and
testing generally. Quite a few supplies
of this sort have been described.,
The one to be described here does
more with fewer parts than any other
with which the writer is familiar. The
circuit is a new one, devised by A. H.
B. Walker in England.- It is not a
regulated supply, but its inherent
regulation is as good at 50 volts output
as it is at maximum voltage. Output
characteristics are those of a
standard condenser -input supply at all
voltages. Internal impedance at d.c.
is about 500 ohms. hum level 5 to 50
millivolts (depending on load), and
output voltage range 50 to 300 v. Regulation
curves are shown in Fig. 3C.
Basic Circuit
The idea of the Walker circuit is to
use a pair of triode- connected power
tubes for rectifiers. with the grids returned
to a variable d.c. voltage. By
cathode -follower action, the cathodes
are then constrained to assume whatever
voltage level that the grids dictate.
The cathodes feed an ordinary
condenser -input filter. The basic cir-
cuit is shown in Fig. 3A. Only a single
tube is shown, half -wave, for simplicity.
Suppose that the peak a.c. voltage
from the transformer is 300 volts, and
that potentiometer, R, is set at 100
volts above ground. If the cathode of
the tube tries to rise above + 100
volts, the effective grid bias will increase,
eventually cutting the tube off.
If the cathode potential falls below
+ 100 volts. plate current will increase.
Fig. 3B shows a 60 -cycle voltage
wave with these levels marked.
The tube can only pass plate current
when the plate is above the cathode
potential, i.e., the potential determined
by potentiometer R through the
agency of the grid. Conduction is
possible during the shaded portions of
the curve.
Minimum output voltage is determined
by the cut -off bias of the tube.
When R is set at zero, this minimum
is generally about 50 or 60 volts. The
higher the mu of the tube, the lower
the "zero" output voltage. For best
regulation, the transconductance of
the tube should be high. Maximum
plate dissipation of the tube is important
if large currents are drawn
at low output voltages.
Complete Circuit
The particular supply shown in Fig.
1 uses the circuit shown in Fig. 2.
Type 6W6 tubes, triode -connected, are
the rectifiers in a standard full -wave
(Continued on page 114)
Fig. 3. (A) Simplified schematic of cathode -follower rectifier. (B) 60 -cycle waveform showing effect of cathode. follower bias. When
bias is 100 volts. cathode "rides" at approximately 100 and conducts only during shaded part of cycle. (C) Regulation curves.
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