Electronics-World-1959-05

www.americanradiohistory.com
WE HEAR a lot these days about the
value of "talk power" especially
from the gang now using SSB. Although
not directly comparable to SSB,
the modulation system to be described
provides a signal with very high effective
modulation percentage and unusually
high "talk power." It is capable
of excellent voice quality, is easy to
adjust, and a single tube such as the
6L6 will fully modulate up to a kilowatt
peak power input.
The schematic of the modulator, Fig.
lA will he recognized as basically the
circuit of the original clamp modulator
first introduced in 1950 as a simple and
economical step from c.w. to phone operation.
Minor modifications of the
original circuit, as shown in Fig. IB, include
use of an ordinary 6L6 in place
of the 6Y6, elimination of the self -bias
resistor which is not required in this
system, and substitution of the vacuum
tube diodes of a 6SQ7 for the crystal
diode generally used.
Similarities bet ween conventional
clamp tube modulators and this system,
however, end at the modulator.
As shown in Fig. 2, modulation in this
system is applied to the screen of the
r.f. driver, or buffer, where the voltages
and currents are of much smaller magnitude,
rather than to the final amplifier
of the transmitter as is usual with
clamp tubes. Other important advantages
in using this system to modu-
A
IeRG_INAL CLAMP CIRCUIT
6Y6
R AS
SIS
LOW -LEVEL CLAMP
CIRCUIT
CURRENT
9F
1HOR
VOLTAGE)
TO SCREEN
OF FINAL A r
POS B NEG
Cr PARTS OF AUDO
c
CYCLE PRESENT
IN DC
la)
Cw
POS PARTS OF
AuD.O CYCLE ONLY
NO NEG PRESENT
TO
OF
SCREEN
BUFFER
LOW CURRENT
B+ (LOW VOLTAGE)
NO BIAS RESISTOR
(BLS MODULATOR AIDS
IN SUPPRESSING POS
CL.. PPE R AUDIO ON ITS GRID.)
(NO BACK -TO -FRONT CONDUCTIVITY,
LESS VARIAT ON IN CHARACTERISTICS)
(GI
Fig. I. Changes made in early clamp -tube
circuit to allow low -level modulation, less
power consumption, and lower voltages.
r
I I I
II
I 1
t!il
late at low level are explained in the
following paragraphs.
Operation of Modulator
Fig. 2 shows the method of clamping
on the screen of the driver, which in
this case is an 807. The d.c. is fed from
a fixed 375 -volt source through the
25,000 -ohm. 4 -watt resistor, R,, to the
screen of the 807 and also to the plate
and screen of the triode -connected 6L6
modulator.
With no audio signal at the control
grid of the unbiased 6L6, the plate and
screen of the tube draw approximately
13 ma. through RI, producing a voltage
drop of 335 volts and thus placing the
screen of the 807 at 40 volts. This, of
course, causes the 807 to operate at
reduced r.f. output which, in turn, reduces
the transmitter carrier output.
On the other hand, speaking into the
microphone produces an audio signal
which is rectified by the diodes of the
6SQ7 and applied as negative bias
(45 v. peak) to the grid of the modulator.
This causes the modulator
plate /screen current to decrease and
the 807 screen voltage to rise, reaching
peaks of 250 volts in accordance
with the amplitude of the audio signal.
Because of the clipping action of the
diode section of the 6SQ7, combined
with the zero -bias operating point of
the modulator, positive peaks of audio
at the grid of the modulator are neg-
By
MARVIN L. GASKILL
W2BCY
Circuit provides high
"talk power," good voice
quality, and is easy
to adjust. Permits single
6L6 to fully modulate
a kw. of peak power input.
Low -Level Clamp Modulator
807
RF DRIVER
FOR PA.
Fig. 2. Triode connected 6L6 modulator is clamped to r.f. driver
screen. Modulator draws current of only 13 ma. on zero audio.
Fig. 3. Preferred circuit for r.f. driver and final amplifier.
.005
A F.
6SQ7
RECTIFIER
2015.
r_+ 40-250 V.
6L6
M00.
NEG. PART OF
AF CYCLE CLIPPED
BY DIODES 8 6L6
FROM
EXCITER
R F
RFC
807
DRIVER
ANT
E
60
AUDIO FROM
SPEECH AMP
(45V)
PHONE
CW
-4SV TO ZERO
25K
4 WATT
RI
+3 5V.
(FIXED SOURCE)
TOOK
R2
50K
GRID DRIVE
-45 V
FIXED BIAS
TO MOD
B+
LINK COUPLING
PREFERABLE
t
FIXED BIAS
REOUIRED
FIKE
SCREEN
ROH
SUPPLY
VOLTAGE
ELECTRONICS WORLD