AM Linear Amplifiers
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Amplitude Modulation
“Incorrectly” in the context below means for linear, non-splattering,
service.
AM linear amplifiers are often operated
incorrectly. This is especially true in CB service, where virtually 100% are
designed and operated incorrectly, but CB operators generally
accept wide signals and poor amplifiers. CB amplifiers are often set up to
process the AM signal and modify the signal like an RF speech processor would.
RF processing makes meters swing, and makes audio sound heavy, but it also
creates very wide bandwidths. This is unacceptable for other applications, where
bands are policed or where people show respect for other users.
The problem with CB amplifiers is that some of
the renegade nonsense theory or improper operation is brought into amateur radio use.
This is especially true when amateurs or Hams use CB linears or CB-design style
amplifiers on amateur bands.
Characteristics of
AM (amplitude
modulation)
Perfect AM in this text is considered to be:
- 100% linear modulation. This means equal positive and
negative peaks- Undistorted modulation
- Steady carrier, not special controlled carrier
systems such as a DX-60 or DX-40 Heathkits
With perfect amplitude
modulation, the signal has the following characteristics:
- Carrier is 25% of peak envelope power. Peak envelope
power/4 = carrier power- Peak envelope power is 400% of carrier power level.
Carrier*4 = PEP- Average power during full modulation is 150% of
unmodulated carrier power. Carrier*1.5 = average power at full
modulation- Carrier power is 2/3 of average power at full
modulation. Average power * .6667 = carrier power- PEP is 8/3 times average power at full modulation.
Average power * 2.6667 = PEP
Based on the above, a perfect 100-watt PEP transmitter would
have:
- 25 watts carrier power, as seen on any type of power
meter without modulation- 37.5 watts average power, as seen on an average
reading power meter with full, steady, modulation- 100 watts PEP, as seen on a true peak reading power
meter with full modulation
Keep the above power relationships in mind for
correct AM linear planning and
operation.
Warning: Tuning for these ratios limits peak envelope power to 100%
positive modulation. If you are operating AM with unknown positive
modulation, or with super modulation, the
TOF
module will warn you of excessive peaks occur.
Linear Amplifiers in AM Service
Traditional linear AM amplifiers are a form of efficiency
modulation. This occurs because supply voltage is constant and does not vary
with modulation. With fixed high voltage, only the supply current
varies with drive.
When we vary current
in a device with fixed supply voltage, the device normally does
not have a square-law power response. We have to change something other than
current to obtain full peak envelope power, since PEP is four times the
carrier power. This is accomplished with changes in efficiency during modulation.
Efficiency modulation occurs naturally in a properly tuned
linear amplifier. The linear amplifier has a constant
anode or collector
voltage. The constant supply voltage means
output device
impedance, or E/I of
the output device,
varies over the
RF audio envelope cycle. The
output device has highest
current on
modulation positive
peaks, and lowest
current on
modulation negative
peaks.
The output device impedance varies over the RF cycle, being
highest at zero power (the full negative modulation peak). Output device
impedance is lowest during the positive peak of modulation.
Because the output device impedance varies over the modulation
cycle, and because the tank or matching system is fixed at one impedance, matching between
the output device
and the load varies
over the modulation
cycle. We want coupling to be perfect at the highest modulation peak, or at the
very highest peak envelope power ever presented to the amplifier. This will
produce peak efficiency during modulation peaks, where a class-B stage can have
over 70% theoretical efficiency (typically it is only around 65%).
The carrier impedance is lower, and amplifier efficiency drops
to about half of the peak efficiency. In practice, with excellent amplifier
design, peak efficiency is around 60%. This places theoretical maximum carrier
efficiency at less than 30%. The reasons are too complex to go into here, but we
really should consider 20-25% as a good carrier efficiency.
This means the amplifier output device dissipates at least
three times the carrier power as heat when a good amplifier is properly cooled,
tuned, and operated.
The following list shows safe limits for properly tuned
amplifiers with different tube types, assuming perfect 100% modulated AM
signals:
All values are per tube with full airflow
| Tube Type | Dissipation | Typical drive power carrier | Typical drive power full modulation PEP | Absolute maximum carrier power output | PEP output power |
| 811A | 65W | 1.5 watts | 6 watts | 15 watts | 60 watts |
| 572B | 160W | 4 watts | 16 watts | 40 watts | 160 watts |
| 3-500Z | 500W | 125 watts | 500 watts | ||
| 3CX800A7 | 800W | 200 watts | 800 watts | ||
| 8877/3CX1500A7 | 1500W | 375 watts | 1500 watts |
This does not mean an amplifier can actually run the above
power levels. The limit for AM power or transmit time is almost always output device cooling. Cooling
is usually planned for noise considerations in amateur amplifiers. This means
tubes will
almost always not take the full absolute maximum power.
The exceptions are with tubes having thin long leads to the
envelope, like the 811A and 572B. The 572 and 811 are designed to be convection
cooled. They do not require forced-air on seals. 572B and 811A anodes
dissipate the same power with or without airflow. The air keeps the envelope and
the surroundings cool. As long as the glass envelope
is kept below approximately 180 F, and as long as external components around the
tube do not have too much thermal rise, 811A’s and 572B’s will handle full rated
dissipation.
This does not apply to glass tubes like the 3-500Z, because
the 3-500Z has significant heat conduction from the anode to the anode seal. The
3-500Z is airflow critical because of conducted heat to the seals through the
very large diameter and reasonably short connections.
Dissipation in tubes with external anodes is directly tied to
airflow, small airflow changes can make noticeable safe dissipation change.
Tuning Procedure
The normal
tuning procedure is
to match the output
device at maximum
positive modulation
peak. This matches the tube to the load at full peak power. As the
modulation positive
peak power is reduced, the
output device has a
progressively higher impedance. This higher impedance from reduced current mismatches
the output tube or output device to the
tank. The result is
a progressive reduction in
efficiency as the
system moves below
the peak positive
modulation level,
reaching minimum
plate, collector, or drain efficiency at
maximum negative
peak when power output is zero.
The above requirement demands we tune or match any linear
amplifier at the absolute maximum peak envelope power that ever appears. If
we tune at a lower level and exceed that level on peaks, the amplifier will lose
peaks. It will become non-linear. The exception to this is if the amplifier uses
a TOF-1 (patent pending) tuning system, in which case improper
operation will show during normal speech operation.
When an amplifier is properly tuned at 100% modulation, and
only the carrier is present, output device
carrier efficiency drops to about half of the
device’s positive peak
efficiency. Let’s assume an amplifier has about
70% anode efficiency,
with 4% tank and
other losses, for 66%
total efficiency. At
carrier levels, plate efficiency will be about 35%. This means on carrier
conditions, 35% of
plate input power
will be lost as anode heat. Tank losses will be constant percentage, at 4% of
the anode RF power. Including tank losses, overall carrier efficiency would be
33.6% with only 1.4% of anode input power appearing as lost power in the tank.
Anode heat will be almost
twice the heat
carrier power
output.
Linear
amplifiers with high
conduction angles
only have about 50%
efficiency on peaks. Along with a normal
design procedure to
slightly
over-couple the output device, some amplifiers will only have around 20% carrier efficiency.
A reasonably safe
general rule
for linear
amplifiers is output
device power
dissipation is three
times carrier
power when
amplifying
unmodulated carriers, although output device heat can be as low as two-times
carrier output power. A
legal-limit AM
linear could have
about 1125-watts
anode dissipation during
carrier conditions
of 375 watts. On
positive modulation
peaks, output power
will be about 1500
watts with 1500
watts of short-term
dissipation. This is
a reasonable safe
estimate.
If
a conventional AM
linear or screen
modulated stage is
making more than
half of the peak
efficiency at full PEP levels when on
unmodulated carrier, odds are
very good the amplifier will have
excessive distortion
and splatter.
Linear Amplifiers on AM, or the
Difference between
Low Level and High
Level Modulation
Low level modulation
often has much less
distortion and more
fidelity than high
level modulation of
tetrodes, and low level modulation more
faithfully
reproduces the audio
input. It is much
easier to have low-distortion high-fidelity audio using
low-level
modulation. To be
sure, some of the
cleanest AM BC
transmitters ever
built were low level
modulated systems.
Unfortunately the
low efficiency
resulted in high
energy consumption,
causing most
stations to use more
energy efficient
high level
modulation.
The sole shortfall
with linear amplifier or
grid modulation schemes is
efficiency. In order
to reproduce the
input faithfully,
the amplifier has to
be loaded to handle
the PEAK power. This
is normally four
times the carrier
power (or more in
some cases). This is
because the linear
has to be
“efficiency
modulated”.
A safe estimate is
25% carrier
efficiency. This
means your amp would
be making three
times the heat as
carrier power. An
SB220 can safely
handle about 500
watts of steady
dissipation
(inadequate airflow
to fully use the
tubes) so it is safe
at 125 watts carrier
when properly tuned.
Very few amplifiers can safely handle legal limit AM. Legal limit AM requires
375 watts of carrier power, and three times carrier power would be a safe
carrier-level heat dissipation estimate.
Typically, with a
375-watt carrier,
over 1100 watts
of heat is produced.
This
takes a lot of air
and a 1200-watt or
higher plate
dissipation tube. An
8877 at full rated
airflow, or a
3CX1200 series tube, would work.
A rig certainly does
NOT need to be plate
modulated to sound
perfect, and as a
matter of fact most
amateur plate
modulated
transmitters have
terrible distortion
as a percentage of
modulation. It’s
just that most
people can’t
actually hear the
distortion, they
listen to and enjoy
the frequency
response and might
actually “like” a
little distortion,
and they confuse
distortion with good
sound.
Contrary to popular myth,
there is no
difference in the
sound of any AM
transmitter when
amplified in a
properly tuned and
operated linear
amplifier. This is
because a properly
tuned and operated
linear, be it a
Heath SB220 or
anything else, has
much less modulation distortion
than the typical
boatanchor rig. The
real problem with a
linear is NOT the
sound. The
real problem is
heat
caused by poor
carrier efficiency.
It’s certainly
possible to have bad
low level
modulation, but
plate modulating a
tetrode also guarantees
we have to do
special tuning and
add “circuit tricks”
to avoid significant
distortion. While
the plate modulated
tetrode system
reduces problems
with loading, drive
power, and heat, it
does not eliminate
these problems.
Additionally,
high-level
modulation requires
a high power
modulation source
with low distortion
and adequate
fidelity.
To be linear all stages must be tuned or loaded at full peak
envelope power, plus a little safety factor. In other words if we are going to
1500 watts PEP output, we must load the amplifier stages to 1500 watts carrier
or more! After loading at full peak power, carrier is set at less than 25% of
the peak power. Failure to do this will result in
modulation distortion called “flat-topping”. The result will be very wide
bandwidth splatter and “downward modulation”.
If we are going to run 100-watt AM carrier levels, all stages
must be tuned for at least 400-watts of peak power.