Amplifier Efficiency and Distortion

Author: N.H. Crowhurst

We have a variety of circuits that can be used to get more power from tubes. Increasing the efficiency is in itself an advantage for many purposes, but it brings with it some disadvantages. Probably the best kind of output for general purpose use, if the inefficiency can be tolerated, is the push-pull triode-operated arrangement. It gives good clean amplification, whether the correct load value (according to design) is used, or a value that may differ from it by a ratio of 2:1 or more.

Plate load can vary from 3000 to 12000 ohms and speaker impedance can vary from 8 to 32 ohms without producing serious distortions.

If, for example, the plate load of the two tubes in push-pull should be 6000 ohms, they will not show serious distortion if an actual plate load is used ranging from 3000 to 12,000 ohms, or even more. In terms of the secondary side of an output transformer designed to feed a 16-ohm loudspeaker, it will not produce serious distortion even if it is connected to an 8-ohm or 32-ohm loudspeaker.

Of course, using an incorrect load in this way will reduce the available output proportionally, but it does not in itself introduce distortion. The maximum rated output will only be given into the correct load but use of a different load does not seriously decrease the maximum output and, provided the amplifier is not run into overload conditions, it will not cause distortion.

This kind of output requires two 25-watt type tubes, which are relatively large, and must dissipate 50 watts of heat in themselves to give a maximum audio output in the region of 6 or 7 watts. If the operating condition changes to class B, the output will be raised to 10 or 12 watts. With this change, the operation condition is more critical. The amplifier needs careful adjustment, but it does not usually introduce serious distortion by using the wrong load value, provided this is higher than the rated value. For example, if the rated secondary impedance of the output transformer is 16 ohms, this circuit will operate successfully with a loudspeaker rated at 32 ohms, but not with one rated at 8 ohms.

The use of pentode- or beam-tetrode-type tubes will considerably boost the available output. A pair of tubes dissipating 25 watts of heat each (a total of 50 watts) can give an audio output of as much as 30 watts. Using this kind of circuit and working into power drive (which means the grids of the output tubes are driven into the positive region at parts of the audio waveform), it is possible to push the power output of two 25-watt triodes working in class-B push-pull up to 50 watts.

Using pentodes this way can give as much as 100 watts front tubes that only produce 50 watts heat between them. The maximum audio output is thus twice as much as the heat generated in the tubes themselves. This is very useful for high-powered systems, but involves critical adjustment, more expensive supply circuits, and careful attention to see that the load value connected to the amplifier is correct. Otherwise, the power output quickly diminishes.

Class B operation can deliver up to 30 watts to speaker. Power drive operation can increase this to 100 watts.

Scope trace is a straight sloping line if output is always a proportionately amplified version of the input signal

What kind of thing happens when amplifiers are not correctly operated? We can see this using the oscilloscope to look at the waveforms. Possibly the most interesting way to do so from the viewpoint of what happens in the amplifier is to connect the input sine wave to the horizontal deflection plates and the output waveform to the vertical deflection plates, instead of using the sawtooth time base. This gives us what should be a straight sloping line on the screen.

If the line curves or squares off in any way, this shows distortion, because it means the output waveform is not following the input waveform in correct proportion. If an attempt is made to get too much power from an amplifier, this means that the grids of the output tubes will be driven positive (or in the case of output circuits designed to take positive grid drive, they will be driven more positive than they should be). The waveform flattens because an increase in fluctuation at the input is not accompanied by a corresponding increase in fluctuation at the output. If a load of an incorrect value is used with tubes that require careful selection of the load, the waveform will change its shape, getting either sharper or flatter towards the tips, or waving about in different ways.

Distortion pattern: left trace indicates clipping, center and right trace indicate wrong amplifier loading.

Distortion due to reactive loading

With amplifiers in which the loading is important, trouble is sometimes caused because the load is not a pure resistance but possesses reactance in the form of inductance or capacitance along with the resistance. In a loudspeaker some of this reactance may be due to the inductance of the voice coil, whereas the resistive component is the d-c resistance of the coil. The actual working of the loudspeaker can cause additional components of resistance and reactance as well. Even if the total impedance of such a combination is the correct number of ohms, it may still cause distortion because the amplifier does not work well into a reactive load. This kind of distortion usually has the effect of "tilting" the waveform when viewed on a sawtooth timebase, or when viewed against the input, it opens out the slanting line trace into a distorted ellipse.