T EVIS i - AmericanRadioHistory.Com

TRANSFORMER SPECIALS
For amplifiers. Utters, exciters.
receivers. etc. Ratings: Pri- -
mary 115V. 50 -60 cycles. Secondary
dzllvers 350 V. from F. W. Rect.) ' a
at 250 MA. Filaments: SV o 3A,
6.3V o 2A and 6.3V p, 8A. Electrostatic
shield bet. primary & '
AG eat Bu y. at Only.'5.75Each 1. '.1 e
H.V. FIL. TRANSFORMER
Stock 38A: Pri: 115V. 60 Cy. Sec: 2 -2.5V
7A. Each tad at 10,000V
DCDHi -Pot. Harm. 5 aleG 2.79
TEST EQUIPMENT
Signal Gen. RCA 710A, 370 -560 MC 350.00
TS 10A Altimeter lest Set 32.50
TS 16 AP Altimeter Test Set
TS 36 Power Meter, 3 CM.
TS 47 /APR Test Osc. So -3000 MC 325.00
TS S6 /AP Slotred Line. 500 MC 325.01
;TS 69 Wagemeter, 340 -1000 MC 72.50
TS 70 /AP Pwr. Meter. 200 -800 MC
TS 110 Echo Box, 2400 -2700 MC
-
SELENIUM
RECTIFIERS
Fuit -Wave Bridge Typos
Current
(Con- 10/14 36/2$ 51/42 130/100
tenuous) Volte Volte Volte V011e
1 AMP. $1.25 $2.11 $3.60 $7.50
2 Amps. 2.20 3.60 6.SO 10.50
2,: Amps. 7.S0 13.00
4 Amps. 3.75 6.75 0.75
5 Amps. 4.95 7.95 12.95 27.11
6 Amps. 5.50 9.01 14.00 33.10
10 Amps. 6.7S 12.01 21.11 11.11
12 Amps. 0.50 16.01 25.50 50.11
20 Amps. 13.25 24.00 30.11 10.00
24 Amps. 16.00 31.00 39.50 98.00
30 Amps. 18.50 36.00
36 Amps. 25.50 45.00
24 VOLT TRANSFORMERS
elFor operating surplus uerate gear. toy trains.
For pe in.
ar. toy 115V. 60
cy.. supplies 24 VAC at 1.2 Amp.. harm.
Baled and cased. A Great Buy $1.49
at Only
RECTIFIER TRANSFORMERS
Pri: 115V. 60 Cy. Sec: 26V, 3.1A, 26V 8.4A
7.3V 14A.. 512.95
Pri: 210'215/220/225 /230 235 240V. 60 Cy., 1
Phase Sec: 11/11/7.5 SVCT p 35A $19.50
Pri: 115V 60 Cy: 1 Sec: 8.1V 4, 1.SA. .51.39
Pri: 115V 60 Cy: Sec: 18.5V e, SA 54.25
SHOCK MOUNT RACKS
FT -156 FT -265A MT -62 ARC -5
FT -162 FT -338 MI -167-U
FT -449
FT -185 FT -487 MT -170A
FT -225 MT -5, ARR -2 MT -171A
MOBILES
C. D. MEN
NEW DYNAMOTOR CONVERTS EASILY TO SUPPLY
12 Volt Input 6V Input
610V o 150 MA OR 300V (. 90 MA
325V p: 125 MA 160V (. 110 MA
Brand New With C sign Data t $3.75
DYNAMOTORS
Input Output Rad..
Type Volta m s Volts Amps Sets
PEn6 28 31.25 250 .060 RC 36
DM416 la 6.2 330 .170 RV 19
DM33A 28 7 540 .250 BC 456
PEIOIC 13 26 12.8 400 .135 SCR 515
6.3 800 .020
BD AR 93 28 3.25 375 .150
23350 27 1.75 283 .075 AN -1
2A0515 12 24 4,2 500 .050
B.19 puck 12 9.4 275 .110 MARK 11
500 .030
0.104 12 225 .100
440 .200
DA -3A 28 10 300 .080 SCR 522
150 .010
5053
P I3CM
28
28
1.4
19
14.5 .5
250 .080
1000 .350
APN -1
BC 375
CW2IAAX 13
26
12.8
6.3
400
800
.135
020
9 1.12
PE94 28 10 300 .200 SCR 522
150 .101
14.5 .5
INVERTERS
PE- 218 -H: Input: 25 26 vac. 92 amp. Output: 115 v.
350 500 cy 1500 voltamoeres. New 544.50
PE -208: Input: 28 vde. 38 mps. Output: 80 800.
500 volt -amps. Dim: 13 "851/ "x
New
522250
LELAND No. 10538: IN: 28 UDC. 12A, OUT: 115V.
115VA, 400 CY 3 PHASE. EXC. COND $70.00
102
In ,. Sr Chock. Shipping Cho. C.O D.
COMMUNICATIONS QUIPMENT CO.
131 L,bvly St. Dept N -4 New York C,ty 7. N. Y.
Childs' Amplifier
(Continued from page 60)
future time. The only shielded leads
used are the feedback loops from the
output transformer to R. and R. All
ground leads are brought to one point
near the 6SN7 tube and grounded to
the chassis. Careful matching of all
paired resistors is important. If the
wirewound resistors are not identically
matched, shunt a small carbon resistor
of proper value across the one having
the highest resistance.
The power supply (see Figs. 7 and
8), is a husky one weighing 83 pounds.
The author wishing to use components
he had on hand rather than purchasing
new ones, selected the large chokes,
CH, and CHI. All components listed
in the parts list are the components
the author actually used, except for
CH,. The 6.3 volt winding on T. had
insufficient capacity to handle all of
the 6.3 volt heaters in the circuit and
for this reason, the 6.3 volt transformer,
T,, was added.
The d.c. heater supply (T4, Fig. 6)
for the preamplifier stage is optional
but eliminates a lot of the hum problem
within the preamplifier. This
heater power supply is not grounded
on the power supply chassis. This
ground is made at the socket of the
12J7 input tube in the preamplifier.
The internal impedance of the plate
supply line feeding the electronic voltage
regulator is 800 ohms and will
supply a voltage varying from 650
volts to 550 volts, depending upon the
current drawn by the amplifier. There
is ample filtering in this circuit as any
hum that remains will be regulated
out in the electronic voltage regulator.
After completing both units the entire
circuitry was thoroughly checked
and met all the requirements desired
by the author. The hum and thermal
hiss was so low that it was necessary
to place one's ear next to the high
frequency speaker to detect it. Using
a test frequency of 60 cycles and
3000 cycles at a 4 to 1 ratio at 30
watts, the intermodulation distortion
was .6%, and with a test frequency of
60 cycles and 7000 cycles at a 4 to 1
ratio at 30 watts, the intermodulation
distortion was .4(4.
The improved damping factor in the
20 to 30 watt range, due to the electronic
voltage regulator, may be seen
by comparing the 50 -cycle square
waveforms shown in Fig. 5.
The proper procedure for making
the final adjustment of the amplifier is
as follows: Before turning the amplifier
on, adjust the bias potentiometer
R. for minimum plate current on the
6J5 drivers. This will increase the
bias on the 807 tubes so as not to draw
excessive plate current until the voltage
regulator is properly adjusted.
R. will be in the position where the
center tap arm is swung as far away
from R:a as possible.
Next, turn on the power supply, al-
lowing ample time for the tubes to
warm up, then adjust potentiometer
R. at approximately its mid -position.
Advance the voltage control potentiometer
R. until an indicated voltage
of 450 volts is read on the voltmeter.
Next, advance the bias control R., until
approximately 25 ma. is read on
the cathode meter MI. At this point
roughly balance the current in both
807 cathode circuits by adjusting R.
and switching S. from left to right
position; when a rough balance is
reached, it then may be safe to decrease
the bias voltage by adjusting
potentiometer R., until 100 ma. is read
on the cathode milliammeter, and at
this point a careful balance should be
made on both cathodes by a fine adjustment
of R:,n.
The internal impedance of the voltage
regulator supply may now be ad-
justed to zero. Move the bias control
R., back and forth momentarily,
thereby increasing and decreasing the
load in each cathode circuit from 75
ma. to 150 ma. as indicated on the
cathode milliameter. At the same
time seesaw the positions of the potentiometers
R. and R,. back and forth
until a point is found where a constant
reading of 450 volts is maintained with
the minimum and maximum current
drain. Readjust R., for an indicated
100 ma. on each cathode circuit. The
final adjustment of the amplifier is
balancing the phase inverter so that
its two plates are 180 degrees out -
of- phase. This is done by placing
switch S, in the first position and
applying a complex wave from a
square -wave generator or record player,
etc., into the No. 1 input and carefully
adjusting potentiometer R. until
no sound is heard in the loudspeaker.
Return S, to one or the other of its
operating positions. Adjust R, in the
power supply to plus 30 to 40 volts
above ground, the reading being taken
at the center tap of potentiometer R.
An adjustment of R, should then be
made to eliminate any trace of hum
which might remain after all other
adjustments have been completed.
In high grade commercial amplifiers
it is a common practice after soldering
joints to cover them with colored lac-
quer. This is done to keep the corrosion
down at the joints, also to show
whether any connection has been tampered
with and to facilitate the identification
of leads. The writer prefers
to apply a wax crayon to the joint
prior to its becoming completely
cooled after soldering. These crayons
offer a large variety of colors and a
fifteen cent box will go a long way.
They have the advantage of keeping
the corrosion to a minimum and if it
ever becomes necessary to unsolder a
joint, it is not necessary to clean the
wax from the joint as the majority
of this will evaporate when the soldering
iron is applied and in no way will
the remaining wax interfere with the
resoldering of the joint. This makes
a neat job and well worth the additional
time spent in the construction
of amplifiers.
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