AM Linear Amplifiers

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"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.