Improper Loading of Amplifier

As far as tank voltages are concerned, there really isn't any difference between excessive drive, load faults, or improper adjustment of loading. Improper tank adjustment and excessive drive are equally harmful to component life, and either can create splatter (and in extreme cases cause keyclicks) on adjacent frequencies.

Most Common Tuning Error 

NOTE: This text assumes your exciter does not have greatly excessive drive power level compared to drive power requirements of your amplifier. If you exciter has significantly more power output than your amplifier requires, you should add an attenuator between the exciter and the amplifier input.  It is generally BAD NEWS to use power controls in radios to reduce drive for amplifiers. This is because MOST  exciters (radios) have ALC overshoot issues.  The overshoot problem worsens as output power is reduced below maximum.

The most common tuning error is adjusting an amplifier at low or reduced drive power as a final tuning step, rather than tuning at maximum obtainable drive. When you load a radio or amplifier at reduced drive as a final tuning step, you are establishing that power level as the absolute ceiling for peak power. Loading at reduced drive sets the loadline impedance at the tube anode too high, and this can cause arcing, splatter, and excessive grid current. 

Always pay attention to factory ratings and never exceed amplifier tuning guidelines. Grid current is especially important to watch, since grids often do not have sufficient thermal mass to absorb large overloads even for short time periods. Excessive grid current can  damage some tubes in less than one second; whereas most anodes will tolerate severe overloads for 15 seconds and longer. It is better to let the anode take the brunt of any mistuning heat.

The last few tuning steps should always be:

• Load the amplifier to maximum obtainable output at full exciter drive
• After that, advance the loading control slightly beyond (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-Drive Linear Amplifiers

Some hobbyists and manufacturers tout "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 situation is one where exciter power greatly exceeds 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.

since July 2004