T EVIS i - AmericanRadioHistory.Com

Fig. 3.
Bottom view of the 30-watt amplifier chassis.
Fig. 4.
Top view of amplifier showing parts placement.
a 4 to 1 ratio, the intermodulation
distortion measured .8 %. As the power
was increased beyond 18 watts the
distortion rose rapidly. This distortion
was caused by a drop from 450 volts
to 350 volts in the plate supply. The
power supply internal impedance was
1000 ohms.
When the power tubes operate in
class A, the plate current does not
vary appreciably and, therefore, the
plate supply voltage is substantially
constant. As the signal is increased
and the tubes function in class AB,,
the plate current is no longer constant.
There will be a voltage swing
proportional to the plate current swing
and the internal impedance of the
power supply. This varying plate supply
voltage then places the quiescent
grid potential in the wrong position
on the "Plate Voltage -Tube Characteristic
Curve" during a portion of the
cycle. The regulation factor of the
power supply causes a voltage variation
to be fed back to the plate transformer
in slightly varying phase relationship
to the signal. This will not
be entirely canceled out by the push -
pull circuit and therefore distortion
will be introduced into the signal.
There will be a loss of power in the
output signal proportionate to the
signal voltage drop within the inter-
nal resistance of the power supply.
To Corr ct these conditions, an electronic
voltage regulator was added
to the entire Childs' circuit, including
the external preamplifier. This meant
an incn'ase in the power handling
capacity of the power supply from 250
ma. to 300 ma. Gr 650 volts and the
insertior of three isolation resistors
and condensers R,:., R,,, R,:, C,, C, and
C:.
The voltage regulator, described by
W. L. K insell in the June 1948 issue
of Rune News, was selected for its
improved regulation over the majority
of regulators using a single pentode
control tube. A quotation in part from
his article is given:
"A method which has been devised
to imprcve the regulation so that the
variations of regulated output voltage
may be reduced to zero or even reversed
is presented herewith. This is
accomplished very simply as shown in
Fig. 2B (shown as R,,:, Fig. 2 in this
article) which adds a potentiometer
or resistor in the screen supply circuit.
This reinforces the regulating action
as described below and reduces the
output v)ltage variations. They can be
made negative if the screen voltage
adjustment is run up high enough.
"Suppose the load current is reduced.
The output voltage tends to
rise, causing the control tube grid to
go in a positive direction. The regulating
action carries through to keep the
output voltage from rising excessively.
"As this occurs. the unregulated input
voltage, E, does rise, due to the
normal regulation of the rectifier and
filter. With RS (shown as R,:, Fig. 2
in this article) in the circuit, the control
tube screen voltage goes more
positive. Since this is the same direction
as the change in control grid
voltage, the regulating action is reinforced
or macle mote effective than
before. It has been found that the
variation in output voltage reduces
as the setting of RS (R,,:, Fig. 2) is
advanced, until it runs through zero
and finally becomes negative. By this
simple means then, the ultimate in
regulating action is achieved."
The ideal setting of this potentiometer
is at a point where the output
voltage from the regulator neither
rises nor falls with the minimum and
maximum current drain of the amplifier.
The approximate setting of R.
is between one -half and three- fourths
maximum. The method of adjusting
this potentiometer will be described
later. The regulated output voltage of
Mr. Kinsell's regulator has been increased
from 350 volts to 450 volts
by the addition of the VR1G5 tube. It
Fig. 5. How the damping factor is improved In the 20 -30 watt range. (A) 50 cycle square wave with a 30 -watt resistive load
without the regulated power supply. (B) 50 -cycle square wave at 30 watts inductive load without regulated power supply. (C)
Same condition as (B) except with regulated power supply. (D) 20 -cycle square wave at 30 watts resistive load with regulated
power supply. (E) 5000 -cycle square wave at 30 wctts resistive load with regulated power supply. (F) 10,000 cycle square wave
at 30 watts resistive bad with regulated supply. (t:) 40.000 -cycle square wave at 30 watts resistive load with regulated supply.
(A)
(B)
IC)
(D)
-.._..1
(E)
April, 1953 59