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Also
See:
Amplitude Modulation
Viking Valiant
Modulation
modifications
Heising modulation, also known as constant current modulation,
was very popular in the early days of AM. It
was largely replaced by
conventional transformer-coupled modulation systems. The Globe Scout series was one of the
very few transmitters still using Heising modulation in the twilight of amateur
radio amplitude modulation dominance. By the mid-1970’s, SSB was rapidly
dominating HF voice operation.
It is important to remember
Heising modulation does NOT use a modulation transformer. Contrary
to popular misuse on AM phone forums and on the air, a dc current bypass choke
or modulation
reactor used in conjunction with a transformer is NOT
a Heising modulation
system! The modulation choke or reactor in a transformer coupled systems prevents power
amplifier steady-state current from flowing in
the modulation
transformer and biasing the modulation transformer core. While this is often
called “Heising modulation” and the choke is sometimes referred to as a “Heising
reactor”, the use of “Heising” is incorrect.
By
definition Heising
modulation has a 1:1
modulator impedance
to PA impedance
ratio. Heising modulation shares power supply current between a PA stage and a
shunt modulator tube. When the modulator tube is biased “on”, the modulator tube
pulls current away from the PA stage. When the modulator tube is biased off,
current is diverted to the PA stage. Power supply load current is nearly
constant throughout the modulation cycle, hence the alternative name
constant current modulation.
Here is the Heising
modulation circuit in my modified Globe
Scout 65A.
How Heising
Modulation Works
Using the circuit
to the left as an example, we can see why Heising modulation is
simple, but limited
in performance.
Heising modulation operates in the following manner:
The modulator tube
(V1),
like any
conventional power
amplifier tube, acts like a time-varying resistance. The anode-to-cathode
resistance pulls the voltage at point A up and down at an audio rate. The large
impedance or inductance of L1 allows
the voltage at point
A to move.
L1, being
a large reactance,
stores considerable energy.
During positive
going modulator
plate excursions
(negative grid
peaks) L1’s
collapsing magnetic
field pushes voltage above B+ rail.
L1 behaves like a flyback
transformer.
The collapsing
field can never
force the voltage up as cleanly or
as far as a regular transformer-based push-pull
modulator. This is
because of flux
leakage and losses
in the Heising
modulation choke.
The best type of
choke would be one
with the lowest flux
leakage. This is not
the type we normally
find or want in power
supply filtering.
Power supply filter
chokes normally have
an air gap in the
laminations. This
gap is created by
stacking the
laminations all E
with E and all I
with I. The more
desirable stack
in a modulation
choke would be an
alternated E and I
position. This
minimizes air gaps
and significantly
reduces flux
leakage. If you look
carefully at power
transformers you
will see they use
alternating stack
positions between
the E’s and I’s. Filter chokes,
for the most part, run all I’s at
one end of the
lamination stack and
all E’s at the
other end of the core stack.
Another way to view
operation is the modulator
tube (V1) and PA tube
(V2) share
current through L1. A basic inductor property is the inductor tries to
readjust voltage
to maintain constant current
flow. While capacitors try to maintain constant voltage across
capacitor terminals with varying current, opposite behaving inductors try to maintain
constant current through terminals while allowing voltage to vary across
terminals!
When modulator tube current decreases, the collapsing field in L1 causes an increase in
voltage at point
“A”. The PA tube has higher operating
anode and screen voltages. Current that formerly went
through the modulator tube now goes through the PA tube, along with the higher
voltage.
When the modulator tube draws increased current, the choke tries to
oppose the current change.
Because of
increasing current,
the choke generates a counter
electromotive force.
This voltage opposes
modulator and final
amplifier supply voltage. In
this case PA and modulator
tube voltage is reduced.
Current that was
flowing through the PA
tube is
now diverted to the modulator tube.
This
is why Heising
modulation is called
constant current
modulation.
Current from the
supply is constant.
The supply current
simply shifts back and
forth between the
modulator tube and
the PA tube. There
is one more
important
consideration and an
interesting
exception to rules,
the modulator tube.
Current into the
PA/modulator section
in a plate modulated
Heising has to be
constant. This means
the modulator tube
has to be class A,
and biased to idle
near or even above
the plate current
into the PA section.
The modulator tube
has to draw at least
as much current as
the PA section to
provide linear
modulation.
There exception
to this is with a
tetrode tube.
Because the tetrode
is combination
screen and anode
modulated, we do not
need to double anode
voltage for 100%
positive peaks. We
also do not need to
take the anode to
zero volts for 100%
negative peaks. In
the Globe Scout,
zero envelope is
reached with 30-35
volts on the 6146
anode. 100% positive
envelope peaks are
reached at about
185% of resting
anode voltage. The
6L6 can be biased to
about 70% of the
6146 current with
minimal distortion,
because both the
6146 anode and
screen are
modulated.
Linearity and Distortion
The modulator tube and
modulation choke must never be allowed to go
into non-linear regions. The resulting harmonic and intermodulation
distortion will appear (unfiltered) as modulating voltage and cause splatter.
This is true even if
the percentage of
modulation is well
below 100%.
One
test of proper linearity is to measure current from the supply through the
modulation choke, L1. If
power supply current flowing through L1 changes
at an audio rate, the modulation cannot be linear.
This is the basis of
constant current
modulation. Of course we can
and we should use
conventional distortion measurement techniques
to measure
distortion, but
constant current is a novel characteristic of
perfect Heising modulation.
There are also
limits in anode voltage
swing on the RF
power amplifier
tube. Even
with with positive
grid voltage causing
a fully saturated modulator tube,
the modulator can never
pull voltage to
zero. Voltage at point A
can never lower than positive voltage at point C. As a matter of fact, the lower
voltage limit at point A on
negative cycles is always greater than the positive voltage available at point C.
This limits modulation to less than 100%, unless a dropping resistor is included
somewhere between the modulator anode and the PA voltage feed. The resistor can
be at any point that limits anode to cathode voltage in the modulator to a
value significantly less than the lowest voltage obtainable at the choke and
modulator anode connection junction with full linear positive-going modulator
grid voltage.
Modulator current flows through
390 ohm cathode bias resistor
R101. Any voltage on
the cathode, even
steady bias voltage,
reduces negative modulation peaks.
It is imperative
to bypass the
cathode bias resistor
R101 with large
value bypass
capacitor C202.
C202’s reactance
must be low compared to
R101’s resistance.
Without C202 across
R101, the cathode will
lift on negative modulation peaks (when the grid of the 6L6 goes positive).
This is a downfall
of the original
Globe system. Grid
bias of the
modulator tube would
have been best.
Lacking that, they
also never included
a low-reactance
cathode bypass
capacitor. Omitting
C202 only made an
undesirable
situation much
worse.
Since modulator and PA anodes are in parallel for
audio signals, the anode voltage swing in the modulator and PA track each other.
This means the 6146 PA has voltage swing limitations similar to the modulator
tube. Because of the
lower voltage limit established at point C
and point A during
positive grid
voltage swings, the 6146
anode can’t possibly
reach zero voltage
on negative
modulation envelope
peaks. The solution is the addition of R106 and C110.
Resistor R101 causes the anode of the 6146 to operate at reduced voltage.
C110
provides a low-impedance path for audio variations. Both
are necessary to
achieve 100%
negative modulation
peaks.
Modulation
Problems with
Tetrode Power
Amplifier Stages
To 100% plate
modulate any power
amplifier RF output
must follow the
square of the anode
voltage. If the
resting anode
voltage is 1000
volts and the
carrier power is 100
watts, the net RF
output power must
reach 400 watts when
the anode is at 2000
volts. Output must
reach zero watts
when the anode
reaches the full
negative peak. This
only occurs when the
plate current tracks
the plate voltage
change. Doubling the
voltage doubles the
current, and this
quadruples the RF
output power. The RF
load impedance also
stays uniform;
voltage and current
are changing in the
same proportion.
This condition of
perfect AM, with
only plate
modulation, can only
occur in deep class
C low-mu triodes or
FET’s that are
neutralized or that
have insignificant
driver feedthrough
power. Such devices
will have square law
power output as
supply voltage is
modulated.
Anode
current in tetrodes
is largely
controlled by the
screen voltage.
While anode voltage
can be run up and
down, anode current
will not
proportionally track
the changes in anode
voltage. A tetrode
will not have linear,
low distortion
modulation unless we
also properly
modulate one or more
of the grids. In
other words we must
use a combination of
grid and plate
modulation. (We
could also correct
the problem by
modulating the
driver stage along
with the PA anode.)
For 100% linear modulation
in a tetrode stage, the screen must
follow anode modulation
in the correct
proportion.
This can be accomplished by operating the screen from the same
modulated source feeding the
anode.
Correcting
Screen Swing in the
Globe Scout
Once again we can
look at my Globe
Scout schematic.
C113 increases audio voltage applied to the screen. In
some radios it might
be necessary to add
a series resistance
with C113, but best
modulation linearity
and lowest
distortion occurred
in my Scout without
a resistor in series
with C113. This
system worked best
with full modulation
swing on the 6146
screen.
In other
cases screen resistor R107
may or may not require
shunt capacitor C113. If C113 is required to
achieve 100% linear modulation, the screen’s audio
voltage can be limited with an additional resistance (not shown) in
series with C113. (Rather than adding a resistance in series with C113,
the resistance of R107 could be split between two series resistors. C113
could then only bypass one resistor. R107 could be a tapped adjustable resistor
of the proper value to establish proper screen voltage, with C113 between the
slider and the end.) Any
resistor in series
with C113 should be
selected for best
modulation
linearity.
My
Globe Scout
The circuit above is the actual
circuit used in my modified Globe Scout 65A. C113 required a large series
resistance in my Scout, about 22K
ohms. This resistor
is NOT shown on the
schematics above.
The resistor value
was determined by
watching modulation
linearity. I use a
sine-wave audio
generator, comparing
the diode-detected
audio of the RF
output to the
generator audio
output. A
standard trapezoidal
pattern would work
also. Remember, the
resistance in other
systems might be
different.
R101 sets the quiescent current of the 6L6 at the maximum allowable
dissipation of the tube. I used a 6L6GC, which has higher dissipation than a
regular 6L6. I adjust R105 to set the 6L6 at the maximum dissipation limit.
I
generally operate the 6146 PA at the same current as the 6L6
while operating AM. This modified system allows 100% negative peaks and nearly 100% positive peaks with
voice. PA current is about 55-65 mA, or 30 watts. In class C
operation, my Globe Scout runs about 18-20 watts carrier output.
Notice
R109. This resistor is a 2 watt high voltage type resistor that connects to the
cathode of the 6J5 triode in the audio amplifier. The bypass capacitor is
removed by snipping the white wire at the cathode pin. The 6J5 is a weak link in
the Globe audio system. Bias is too low. On positive peaks, the 6J5 grid
actually goes into conduction. This clips the signal from the gain control, and
limits positive peaks.
R109 does two things. It biases the 6J5 closer to
cutoff, allowing the grid to swing more positive before drawing current and
limiting. It also adds negative feedback, reducing non-linearity in the 6L6
modulator tube.
I tried removing R101 and using grid bias to reduce the
lower-voltage limit (because point C could be chassis ground), but the actual
gain in negative peak linearity was too small to make the mod worthwhile.
I
also added a grid leak resistor, a screen clamp set at 225 volts, and a fixed
grid bias source. The fixed bias is set so the Globe draws about 40mA with
HV and no excitation. It is important you do this, because the original 6146
cathode resistor of 450-ohms adds negative bias that decreases modulation level
and linearity. If you want to leave the 450-ohm resistor in, I suggest adding an
electrolytic capacitor for audio frequency bypass. In that case it would not be
harmful, and you could omit C110 and R106. I have not tried this because I
converted my Globe to grid block keying and eliminated the 450 ohm resistor long
before starting audio modification.
With no other mods the Globe Scout
is flat from 200Hz up to about 3500Hz, where it starts to roll off. By 6000 Hz
audio is 6dB down. The Globe Scout audio is comparable to any other good radio
(Ranger, etc.) on AM.
This is the original Globe Circuit. (The penciled-in components were from a
mod I made in 1963) :
 Since creation 2004 9/26
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