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Tuning an amplifier

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Tuning an amplifier

Post  KG4BPV on Sun Feb 05, 2012 9:54 am

Here is an email I got from a friend re: loading an amplifier. It is in-depth! It's an article from the web, sorry but I do not know the source.

Important!!!!!

In grounded grid amplifiers, the single most important meter to indicate proper final operation is the grid current meter!! Grid current is far more important than anything else, with output power being second only after grid current.

In grid driven tetrodes or pentodes, the most important parameter is screen current with control grid current second and output power a close third.

Note: An inexperienced tuner must have a power output meter to ensure proper tuning.

There are two destructive things that can happen when we mistune an amplifier, heat and arcing.

Heat is always a time function that depends on the thermal mass of the thing being heated and how much heat is being applied.

Arcing is an instantaneous problem. Arcing can destroy things in fractions of a second!





Basic Operational Theory (this section is optional reading SKIPDOWN)

The output device in your amplifier has a certain optimum available voltage swing, and it has limited current available. It is important that the load impedance presented to the output device matches the optimum values of available RF voltage and current. When we adjust the tank circuits (or auto-tuners) in our power amplifiers (PA), we are really setting or adjusting the load impedance presented to the output device. Here's what happens when we tune:

1.) If load impedance presented to the output device is too low, current is excessive and efficiency suffers. This is also called over-coupling. This causes too much heat. Heat is a long term problem that takes a finite time to cause damage. It is generally NOT instantaneous damage, although tube anodes or transistor junctions can be overheated to the point of damage in a matter of 15-30 seconds in some cases. This is the case where we use too little loading capacitance.

One good thing about over-coupling is screen or control grid current is reduced, and this protects the most sensitive and easily damaged parts of vacuum tubes. Another advantage in tube amplifiers is linearity generally is a bit better with slight over-coupling. There is slightly less splatter or distortion.

2.) If load impedance presented to the output device is too high, current is reduced but voltage will be too high. This is called under-coupling. This is the case where we use too much loading capacitance. Efficiency is normally very good, heat is reduced or remains in a normal range for the level of output power produced. Voltage can increase well above the supply voltage limits, up to several times the dc supply voltage in extreme cases. This is the worse scenario because severe damage can be instantly caused by arcing or voltage breakdown of components, and damage can be instantaneous even with very slight over-voltage. Worse yet, once an arc starts, it causes a dielectric failure. The dielectric failure destroys insulation, creates sharp points or surface irregularities that reduce voltage breakdown, or the arc ionizes air or creates a plasma. All of this works to sustain the arc even after voltage is reduced to safe levels.

Under-coupling, or having the loading capacitor closed too far for the load impedance and/or drive power, increases grid current and splatter. It creates a very hard form of non-linearity where the device switches into non-linearity very quickly, and the sharp transition into non-linearity or gain reduction creates a very wide bandwidth splatter.

If we have a coupling error we would like it to be slight over-coupling in the PA output device. It is better to see a little too much device plate, drain, or collector current than too much voltage at reduced supply current. We also do not want excessive grid current in vacuum tubes.

For this reason, almost all "pre-tuned" solid state amplifiers are over-coupled to the load. They are actually optimized for a higher than normal load impedance by slightly over-coupling the output devices to the load.

SWR or Reflected Power Myth:

We often hear people claim reflected power burns up as heat in the power amplifier stage. This is not true at all.

The only effect of reflected power is it changes the loadline of the output device. This can either increase PA device RF voltage swing, or it can increase PA device current. If the voltage increases heat generally is reduced, but the PA can arc. If the load mismatch is of a phase angle that increases current, PA device heating increases because conduction angle and peak current increases.

In one case heat increases, in the other heat decreases. An SWR mismatch only requires the matching network be readjusted to restore the proper loadline at the output device. In an adjustable pi-network or pi-L network system the only effect of SWR is in current in the inductor(s) and voltage across the loading capacitor, so long as the network can be adjusted to proper load at the output device. in other words if you can retune the network and don't exceed voltage breakdown of the loading capacitor, your amplifier is very likely OK for any SWR.

Improper and Proper Loading of Amplifier (read this section)

There is very little difference between excessive drive power, antenna system faults or failures, or grossly improper adjustment of loading. All can be equally bad.

Improper tank adjustment, antenna system failures, and excessive drive are equally harmful to component life. Improper tank adjustment, antenna system failures, and excessive drive either create splatter (and in extreme cases cause keyclicks) on adjacent frequencies, or they cause excessive heat in the output devices or components in the system. Regardless of the reason for them, amplifiers are damaged by excessive tank voltages or device currents caused by improper adjustments that prevent proper energy transfer to a load.

In some cases, particularly on the lower end of the lowest frequency bands, proper loading cannot be achieved.

Signs of UNDER-coupling

When the output capacitor (load capacitor) is meshed too far (too much capacitance), especially at high drive power levels, the amplifier will be under-coupled. Under-coupling is the very worse thing to do to any amplifier because failures can occur in a matter of seconds! There are several signs of under-coupling in a grid-driven tetrode or grounded-grid amplifier. Watch closely for the following:

1.) When the drive power, using a steady carrier, is slowly increased the grid current (either screen or control grid) will at some drive level suddenly rapidly increase. The sudden rapid grid current increase will be disproportionate to the plate current or drive power increase! DO NOT go past the point where grid current starts to rapidly increase with small changes in drive power level.

2.) Too much grid current, either screen or control grid, is a clear sign you have the loading control too far meshed or closed.

In a grounded-grid amplifier or a grid driven tetrode amplifier, the grid current meter (control grid in the triode, screen grid in the tetrode) is the most reliable indicator of improper loading and/or tuning. Be especially watchful of disproportionately high grid currents compared to anode currents or drive power, or a rapid increase in grid current with a modest increase in drive power.

Never tune, peak, or dip the amplifier at reduced drive power, and then attempt to operate or attempt to suddenly apply full drive! If you are going to make a mistake, make the mistake by having the loading control too far open or unmeshed...not too far closed or meshed! At least with the loading control too far open, you will not cause an arc, blow out a bandswitch, or damage a tube grid. You have slightly more time for mistakes and corrections when the loading capacitor is open too far than too far closed.

Most Common Tuning Error

Too much grid current is almost always a sign of a loading control that is meshed or closed too far for the amount of drive power. This is hard to see on SSB, and best to view on CW.

NOTE: This text assumes your exciter does not have greatly excessive drive power level compared to drive power requirements of your amplifier. If your exciter has significantly more power output than your amplifier requires, you really should add an attenuator between the exciter and the amplifier input. Using power controls in most radios to reduce drive more than 50-70% for amplifiers is generally a bad idea. This is because many exciters (radios) have ALC-overshoot issues. The ALC or power overshoot problem worsens as output power is reduced below maximum.

There are exceptions. The Yaesu FT1000/ FT1000D has a drive control and a power control that functions in all modes. Backing the drive control off so ALC is barely registering assures there is no ALC power overshoot. On the other hand some ICOM rigs, no matter how they are adjusted, will overshoot beyond the factory rated power levels. I have an IC-706 that will overshoot to 130 watts or more when set at any power level, even 20 watts! I had an IC-775DSP that would go over 200 watts of very short RF peak output power when set at 75 watts. These radios, or other radios like them, can trigger arcs in amplifiers and are generally rough on components.

The most common amplifier tuning or loading error is adjusting an amplifier at low or reduced drive power as the final amplifier tuning step. When we load a radio or amplifier at reduced drive as a final tuning step, we are establishing that power level as the absolute ceiling for drive and output power. Final loading at reduced drive results in a loading control too far meshed. This can cause arcing, splatter, and excessive grid current.

Ideally (if possible) we should make the final tuning and loading adjustments at or near maximum exciter drive power. Some amplifiers drive too easy to do this, so we should always pay attention to factory instructions and avoid exceeding factory amplifier tuning current limits, especially for control and screen grids. Grid current is especially important to watch because grids often do not have sufficient thermal mass to absorb large overloads even for short time periods. Excessive grid current in metal oxide cathode tubes (ceramic tubes with indirectly heated filaments) like the 8877 and 3CX800A7 can damage tubes in less than a few seconds; whereas most anodes will tolerate severe overloads for 15 seconds and longer. It is better to let the large anode or plate in a tube take the brunt of any mistuning heat, which means with any mistake it will be better to over-couple or have the load control capacitance slightly lower than optimum.

The last few tuning steps should always be:

Load the amplifier to maximum obtainable output at full exciter drive (without exceeding amplifier short term overload ratings)
After that, advance the loading control very slightly beyond that point (towards less capacitance).

ALWAYS load your amplifier for maximum obtainable power, and reduce drive to rated, safe, or desired operating power levels! This ensures minimum voltage and current in the tank and maximum possible linearity (best signal quality). High grid current is a strong indicator of excessively light loading in grounded grid amplifiers.

Exciter Transients or Power Overshoot

Maximum available carrier drive might not result in sufficient drive for tuning. This is especially true when an exciter has transients or power overshoot from marginal ALC response.

Transients or overshoot appear on the leading edge of the RF envelope, on the leading edge of speech or CW transmissions. This is the time when the transmitter is going from zero power towards full power. Since the ALC circuit has no stored voltage at this moment, the exciter runs full throttle for an instant. This effect is missed by most power meters.

Once the ALC comes up, the hang time of the ALC will hold the exciter gain back. Transients and/or overshoot will generally disappear.

Transients and overshoot, being of short duration and infrequently occurring, make it impossible to tune correctly at maximum drive. With transients or ALC overshoot, it is impossible to tune your amplifier properly by simply tuning for maximum output with a carrier, a tuning-pulser, a whistle, or normal speech. We cannot just tune for maximum output and expect the amplifier to be properly loaded when the exciter has leading edge ALC transients!

Let's assume the exciter is rated to deliver 100 watts, but has momentary peaks or transients of 160 watts while the ALC or power control loop "takes hold". Power surges of 160 watts, too short to register on normal power meters, occur at the start of every transmission. Of course, if we don't run the exciter wide open and reduce power to 50 watts the problem actually gets worse! In this example the transient peak would still reach nearly to the same 160 watts, but the amplifier would be tuned for 50 watts drive! This is bad news for splatter and for components in the amplifier.

This is why the maximum power setting of the exciter should generally be used while tuning. If the exciter has far too much drive for the amplifier, we need an attenuator or an amplifier better matched to the exciter.

The loading control should always be advanced a reasonable amount beyond (further open) the actual maximum output power setting. This will allow the amplifier tank system to handle transients without arcing or component failure.

Easy-to-Drive Linear Amplifiers

Some hobbyists and manufacturers tout "very low drive" as an advantage, claiming it offers "cleaner signals". Nothing is further from the truth.

Exciters almost always provide the best IM performance when operated at a time-averaged peak power a reasonable amount below full output, rather than very low levels. At low power levels, exciter performance is dominated by cross-over distortion. This is where bias non-linearity or device input threshold induces distortion. The ALC system also adds cutoff bias to early stages. This bias increases distortion in ALC controlled stages. At very high levels, gain compression or negative bias shift becomes an issue. Exciters typically do best when operated in the area of 60-80% of rated power.

Worse yet, low drive amplifiers are especially susceptible to damage from exciter overshoot or transient problems. Transients and overshoot peak power remains almost the same level regardless of exciter power control settings. As exciter operating power levels are reduced, the percent of power overshoot becomes worse.

The most undesirable situations are those where exciter power greatly exceeds (by more than twice) an amplifier's normal drive power limit. Not only does this reduce system IM performance, amplifier drive transients are aggravated. Amplifiers should be designed or selected to match the exciter's maximum power output, or an external attenuator used to bring the amplifier's drive requirement up to the exciter's full power level. Low drive amplifiers are, as a general rule, bad news.

Amplifiers Without Enough Loading Capacitance

Some amplifiers do not have enough loading capacitance. The loading or antenna coupling control is all the way at maximum (capacitor fully meshed) for maximum output power, making it impossible to "peak" the output. Opening the loading capacitor up more just reduces the output power, no matter what the drive level. This is over-coupling that cannot be corrected. It can be caused by several things:

1.) The loading capacitance is inadequate through bad or improper design. This is common on the lower end of the lower bands in some amplifiers. For example the Kenwood TL-922 (which works better in the old Japanese segment of 160 meters, above 1900 kHz). Another amplifier that had poor tuning range on 160 and 80 meters was the original AL80, the Dentron Clipperton, and several Amp Supply amplifiers.

2.) The output power level you are tuning at is lower than the design target. As power is decreased, the maximum-power-output loading capacitance setting always increases. In other words as drive is reduced and we re-tune, the output power "peaks" with more and more loading capacitance.

3.) A padding capacitor has opened up.

4.) A tank inductor has shorted between turns or does not have enough turns. (Common in Dentron amplifiers on lower bands, where loaded Q is often 20 or more.)

5.) Antenna system impedance at the amplifier is too low, or is slightly inductive rather than being resistive or capacitive.






KG4BPV

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