T EVIS i - AmericanRadioHistory.Com

excess of the manufacturer's rating
for either the 6J5 or 6SN7 tubes.
However, the plate dissipation is not
exceeded. The actual voltage applied
to the plate is the 450 -volt plate supply
plus the minus 35 volts that the
cathode is held below ground by the
bias supply, making the total 485 volts
applied to the plate. The author has
used 6J5 tubes under these conditions
Fig. 2.
in other circuits for at least three
years, and has never encountered any
difficulty with the life of the tube or
the functioning of the circuit. 6S4
tubes could have been substituted for
the 6J5 tubes without exceeding the
manufacturer's rating. The 100 percent
degenerative feedback within the
cathode follower circuit and its low
output impedance make an ideal
Schematic and parts list covering the modified Childs' amplifier.
R9
6A5.4 5
6557
6517
6J5
645
6J5
6J5
6547
R,, R_, R,4. R,- 220.000 ohm, 1/2 w. re s.
R:,, R,, -1500 ohm. 5 w. wirewound res
R, -2000 ohm wirewound pot.
-
R,,, R -1250 ohm, 5 w. wirewound res.
H,. R 100.000 ohm. 2 w. res.
58
R,., R,.- 22.000 ohm. I w. res.
R,,,. R ,,- 100.000 ohm, 1 w. res.
R,:, -2500 ohm. S w. wirewound res.
R,,,, R,7, R,-_- 50.000 ohm, 1 w. res.
R,,,, R22-2400 ohm, 1 w. res.
R_.,- 100,000 ohm. 'r2 w. res.
R2-3000 ohm, 5 w. wirewound res.
R_, -7500 ohm wirewound pot.
R, R2.,-23.000 ohm, S w. wirewound res.
R,-5000 ohm. S w. wirewound res.
N R;,,. R,,. R:., -1000 ohm. 1': w. res.
R. R,61. R:,7. Ro, -20 ohm, 1 w. res.
Rs_. R:,,. R5.-100 ohm. I w. res.
R:4 -50 ohm, 4 w. wirewound pot.
R,,, R.2. R4,. R4,. -47 ohm. 1 w. res.
RIB
R,::. R,4. R47. R:,r100 ohm, 12 w .
R,;,- 10.000 ohm, 10 w. wirewound res.
R ,-10,000 ohm, 10 w. wirewound pot.
R:4- 470,000 ohm, i z w. res.
R :4,-100.000 ohm pot.
R :4- 27,000 ohm. 1/2 w. res.
R,s :. R:..,-Use correct value to calibran
meter to full scale, 6 db. 16 db, 26 db..s.,d
36 db
807 ' R3B Rag 807
R37
F3
R40
rái
R41 R42 R48 R49
2 s 6A57G 2 s 61457G
R47 550
R431 ke 4 "545
R51
R52
TC8
R46 Ire 1 553
X X
L. SJ7
VR75 5
R54
144
VR75
s
VR75el
2
e
C,. C_, C -20 pfd.. 600 v. elec. cond.
Cs. C C.-,. C,,-.1 pfd. oil -filled bathtub cond.
pfd., 600 v. mica cond.
S, -S.p 3 -pos. wafer switch
5.-4 -pole d.t. wafer switch
5-pos. wafer switch
F, -1/4 amp. /use (Littelluse)
8,-1/2 amp. fuse (Littelluse)
F,- 1,r amp. fuse (Littelluse)
F,, F -1/32 amp. fuse ILittelfuse)
T,- Peerless 265Q connected 2500 ohm primary,
16 ohm secondary
M, -0.200 ma. d.c. meter
M. -0.100 ma. d.c. meter
M:, -0.500 volt d.c. meter
M4 -10 + 6 db meter
1 -6SN7 tube
1 -6SL7 tube
4 -6Ji tubes
4 -807 tubes
2 -6AS7G tubes
1 -6SJ7
3 -VR75 tubes
Note: All fuses, S2, Sa, and R:,.; mounted on
meter panel.
direct -coupled driver for the nxed bias
operation of the triode- connected 807
tubes.
At this point, the 807 tubes were inserted
in their sockets and the feedback
loops were disconnected from
ground and connected to the voice
coil. A sine wave was applied to the
input of the amplifier and a measurement
was taken from ground to each
half of the voice coil winding to determine
the uniform balance of the
push -pull circuitry.
Two husky broadcast -quality transformers
were tried; both of these
transformers had a primary impedance
of 2500 ohms and a tapped 500 ohm
secondary in addition to the 16 ohm
voice coil winding. These transformers
were good transformers and had
worked very satisfactorily in other circuits.
However, due to the stray
capacitance caused from the unused
500 ohm winding, it induced
sufficient phase shift to give an undesirable
unbalanced output over the
entire frequency range. This fact is
mentioned not to criticize the transformer
quality, but to emphasize the
necessity for having a transformer
that is properly designed for this par-
ticular type of circuit. The Peerless
S265Q transformer was used and found
to give a highly uniform balanced output
on both halves of the secondary
for a frequency range of 15 cycles
to 70,000 cycles. Obviously there are
other transformers that would work
just as well. However only one was
actually tried.
The plates of a cathode -ray oscilloscope
tube were connected directly
to the voice coil winding with a 15
ohm resistive load shunted across this
winding. The oscilloscope amplifier
was not used in order to eliminate its
distortion from the patterns observed.
A 15- cycle, 30- cycle, and 60 -cycle
square -wave signal was applied to the
input of the amplifier, and at 18 watts
a slight phase shift in the output was
observed. There was no overshooting
or hangover effect observed in the pattern.
The square -wave signal was then
increased to 10,000 cycles in steps of
500 cycles. Through this entire range
at the 18 watt level there was little
noticeable hangover or overshooting
observed in the pattern. Very straight
sides and no rounding of the corners
were seen. For curiosity sake only the
square -wave generator was then increased
to a 30.000 cycle fundamental
frequency and it was observed even at
this phenomenally high square -wave
frequency the pattern still resembled a
square wave. These tests showed that
the amplifier was highly stable. had
excellent transient response and had
a well damped output with a nearly
flat frequency response from 15 cycles
to over 100,000 cycles. An intermodulation
distortion measurement was then
made of the entire amplifier at an 18
watt output. Using a test frequency
of 60 cycles and 3000 cycles at a 4 to
1 ratio, the intermodulation distortion
measured 1.5;:. Using a test frequency
of 60 cycles and 7000 cycles at
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