T EVIS i - AmericanRadioHistory.Com

I
AN ALL
By E. R. GAINES, WSUW
USH-PU IL
AMPLIFIER
Method of providing the requisite out-of-phase voltage
without resorting to usual vacuum -tube phase inverter.
VACUUM tube phase inverters are
very popular systems for obtaining
out -of-phase
voltage for driving
push -pull audio amplifiers. Their
shortcomings may be one or all of the
following: low gain, unbalance, distortion,
and the fact that any hum introduced
in the circuit prior to or at
the phase inverter is fed out -of-phase
TO AMC. RECTIFIER
1OOppla.
I
TRANSFORMER
IN34 01
to the grids of the push -pull output
stage resulting in 60 or 120 cycle output.
An excellent remedy for this headache
is the elimination of the tube -
type phase inverter and the utilization
of push -pull stages throughout the
amplifier. Out -of -phase voltage is obtained
by applying the audio signal to
REAR VIEW
CRYSTAL
CARTRIDGE
SHIELD CONNECTED I
TO CARTRIDGE SNELL I
AND AMPLIFIER
CHASSIS
(a)
II I
I
I
'
I
I
I
I
t
111 BROKEN
GROUND STRAP
Fig. 1. (A) Detector circuit to permit radio to drive any amplifier with push -pull
feature. (B) Driving the amplifier by means of a crystal pickup uses this circuit.
Fig. 2.
PUSH -PULL
INPUT
RI
R2
Circuit of the author's experimental amplifier using dual potentiometers.
lt
It
RIE
RS
C3
Vt
ilrlrll
65N7
65L7
C2
kid
RB
b TO
OVOICE
COIL
opposite ends of a centertapped resistor
or alternately to a dual potentiometer,
which also serves as a volume
control. Apart from the excellent
balance obtained (which is dependent
on the precision of the dual potentiometer),
the push -pull stages convey
the hum picked up in the circuit to
the grids of the output stage in phase
and the 60 or 120 cycle component
has a pronounced tendency to balance
out in the primary windings of the
output transformer.
Fig. 2 is a diagram of an amplifier
constructed by the author using a
potentiometer phase inverter and
push -pull stages throughout. It is indeed
a high -fidelity amplifier which is
relatively inexpensive -but the most
interesting results observed were obtained
when the gain control was set
"wide open" and the ear was placed
near the 15" speaker (bass reflex en-
closure)-no hum could be heard! Try
this on any amplifier. Good results
could be obtained without any shielding
at all -just twisted hookup wire
running from the audio input source
to the amplifier; however, shielded
wire is recommended at this critical
point.
A detector to enable the builder to
use a radio to drive any amplifier with
push -pull input provisions is shown in
Fig. 1A. A crystal pickup can be used
to drive this amplifier provided neither
of the two "hot" terminals is grounded.
(Fig. 13). Many of the later model
cartridges are grounded externally by
means of a clip and present no problem,
while some of the older crystal
cartridges have the ground strap located
in a conspicuous place and it
can be removed easily.
Tone controls present no problem
except a slight increase in cost. Duplicate
halves are required for each
side of the circuit and dual potentiometers
should be used. No tone controls
were used on the experimental
amplifier in Fig. 2 as the response
-
was
essentially flat from 20 to 20,000 cycles
and the reproduction sounded excellent.
Fig. 3.
Circuit diagram of the power supply.
DUAL L MEG POT
+3OOV.
R, -R-Dual 2 megohm pot. (See text
R,. -4700 ohm, 1/2 r. res.
R... R.- 220,000 ohm. 1V2 r. res
R- 10,000 ohm, 1 r. res.
R.. R,,- 500.000 ohm. V. r. res
R.,, R.-1500 ohm. 1/z . res.
R,,- 47,000 ohm, 1/2 r. res.
R,,. R, -- 270.000 ohm, '_ . res.
F.+
1 yt A
V2 F R16
6.3V.AGEA.
E.SV.AG CT
R,a -330 ohm, 10 r. rireround res.
C, -10 pfd., 350 v. elec.. cond.
C_. C11. Cs. Cs-.1 ,std.. 400 v. cond.
T,-- Output trans. 5000 ohms plate -to -plate to
v -c.
V, -6SL7 tube
V.. -6SN7 tube
V;,, Vs -243 tube
(. -8 pfd, 450 v. elec. cond.
C -40 pfd.. 450 v. elec. rond.
F, -117 r line fuse
T, -Pore trans. to supply 250 to 300 v. d.c.
@ 200 me.; 6.3 v. centertapped @ .9 amp.;
S r. @ 2 amps.
trans. 2.5 v. centertapped @ 5 amps.
(used for 2A3's)
CFI,. CH.,-8 by., 200 me. filter choke (Merit
C -31961
V, -5V4 tube
78
RADIO & TELEVISION NEWS