Electronics-World-1959-05

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type of phase -splitter, the signal voltage
across the plate and cathode resistors
is essentially of the same magnitude
as the input voltage to the tube.
Normally, this would require a large
voltage from V,. Fortunately, because
the speaker load is also in the input circuit
of the phase -splitter tube and in
the correct sense to provide positive
feedback, a much lower voltage suffices.
In fact, the peak signal voltage to the
input of the phase -splitter need be only
as high as the fixed bias applied to the
power tubes, for maximum output. A
novel feature of this amplifier, as will
be noted in Fig. 1, is that the output
load is part of the input circuits of all
the tubes used in the amplifier for the
purpose of obtaining both positive and
negative feedback. This has been accomplished
without the use of any reactive
components which might produce
undesirable phase shifts.
Amplifier Circuit
The complete amplifier is constructed
on a 13 x 6 x 1% inch chassis.
The schematic is given in Fig. 2
and along with the accompanying
photographs indicates the construction.
A 6AN8 tube is used as a combined
voltage amplifier and phase -splitter.
The unusually large plate load resistor
(1.8 megohms) of the voltage amplifier
produces a gain of over 1000 from this
stage and is entirely practical since the
direct coupling to the phase -splitter
eliminates the usual loading effect of
the following grid resistor.
Each half of the series -connected
push -pull output stage consists of five
type 12B4A tubes connected in parallel.
There are two identical plate current
supplies for this stage, each consisting
simply of a silicon or selenium rectifier
and two 300 pfd. electrolytic capacitors
connected in parallel. The power
transformer is a special low- impedance
unit in which the primary and the 130 -
volt winding feeding the power supplies
for the output stage, each has a
d.c. resistance of less than 1.5 ohms.
This is essential so that the large current
demands of the power tubes, when
peaks of program material are handled,
can be met. Despite the absence of
filter chokes or resistors in these power
supplies, the signal -to-hum ratio of the
amplifier is very high, better than 50
dbm. This is because the amplifier itself
acts as an electronic filter.
To prevent the initial surge of charging
current through the 600 pfd. filter
capacitors (two paralleled 300 Mfd.
a,
r
<
is
Tr
N
70
R,- 270.000 ohm. 1/2 w
Rs, Rs, R,e, R,,, R.,. R.,. r Rn. R,.. R,,. R,,.
100 ohm, 1/2 w. res.
R.+ -5100 ohm, 1/2 w. res. ± 5
R, -1.8 megohm. 1 w. res. ± 5';
Rs -750 ohm, 1/2 w. .
Rs -8.2 megohm. 1 w. res. w 5';
R,. Rs- 18,000 ohm, 2 w. res. (matched with ohmmeter)
Rn. Rea- 47.000 ohm, 1/2 w. res. ± 3';
Rn. L,- 15.000 ohm. 1/2 w. res. ± 5';
Ro- 82,000 ohm, 1/2 r
R.3- 82.000 ohm, 1 w. w 5';
L, 230.000 ohm pot ( "Hum Balancing ")
Ro.- 22.000 ohm. 1/2 r. s.
R,,-6800 ohm. 1 w. res re
Rr.- 10.000 ohm, 1 w. res.
Ro-300 ohm, 5 w. res.
Lis-10,000 ohm pot ( "D.C. Balancing ")
C,-50 pfd.. 6 r. elec. topsoitor
C,. Cs. C4. C.-.22 pfd., 400 r. capacitor
Cs, C.. C,. Cs-300 pfd.. 175 r. elec. capacitor (two
paralleled, see text)
C,.- Cu -C- 15/20/20 pfd.. 350/450/450 r. elec.
capacitor
rrtr
V6
° QI"QI' Q1 QI' QI'
tttretttgr
rq
Q
0
1 S
.,r.c
Cu -C-70/10 pfd., 50 r. elec. capacitor
C,s -.047 pfd., 600 r. capacitor
SR,. SL--500 ma., 130 r. a.e. silicon power recti-
fier
(Stokes Taman M500 or Audio Devices
A750)
SRs-20 ma., 65 r. selenium rectifier (Radio Receptor
4Y1)
J ,-Phono jack
RL,-9000 -ohm relay (Sigma type 11F)
S,- S.p.s.t. switch (see text)
SO, -Octal socket (to match preamp used with this
amplifier)
F,. F.-1 amp fuse (Slo -Blo type)
F. ,-3 amp fuse (510-Blo type)
Ts -Power trans. 365.0.365 r. @ 30 ma.; 6.3 r.
(a .6 amp; 6.3 r. @ 3 amps; 31.5 v. @ .6 amp;
130 v. Ida. resistance of winding to be less than
1.5 ohms), 117 volt primary with the d.c. retsistance
of winding to be less than 13 ohms (see
ext)
V, -6AN8 tube
V,. V.. V,, V. V.,
Vs, Vs, Vs, V,,, V,,-1284A tube
V11-6X4 tube
Fig. 2. Complete schematic diagram and parts listing for the ultra -stable amplifier.
units) from damaging the rectifiers
when the amplifier is first turned on,
a surge -limiting resistor of 300 ohms
(R) is connected in series with the
rectifiers. The function of the relay
(RL,) is to short out this resistor when
the voltage across the filter capacitor
C. builds up to a value high
enough to operate the relay. This usu-
Fig. 3. Power response measurements made with both a 16 ohm and a 32 ohm load.
20 100
22-OHM LOAD
*-OHM LOAD
.c io.C. 204C
FREQUENCY -CPS
ally takes approximately two seconds.
The Sigma type 11F relay used here is
an inexpensive unit selling for $1.80 at
most parts distributors.
Fixed negative bias of 25 volts for
the upper half of the output stage is
obtained from a voltage divider R_,, R
connected across the plate supply of
the lower output tubes. These lower
tubes also have a fixed bias of approximately
25 volts, which is developed
across R:.,. SR., C,., and C,., supply this
voltage. The d.c. balance pot (R) is
used to adjust this bias so that there
is no d.c. in the speaker voice coil.
Plate voltage for the 6AN8 voltage
amplifier and phase -splitter is obtained
from a 6X4 rectifier tube (Va). The
current required is less than 6 ma. and
is well filtered by C,,, C,2, and
Plate voltage for an external preamplifier
is available at the octal socket and
is filtered by C. and R., Most preamplifiers
such as the Heathkit and Dyna-
ELECTRONICS WORLD