PlayJazzGuitar.com Forum

[randyc]

In some of the discussions that I've posted, I've frequently noted that vacuum tube guitar amplifiers are low-fidelity devices. There's little point in designing an amplifier to have the performance characteristics of a high fidelity unit when the instrument that is to be amplified has a fairly limited frequency response (about 80 to 1300 Hz). Even if we include adequate bandwidth to allow the second and third harmonics to be heard - to enable the audible detection of "timbre" - we still require only a frequency response of 80 to 4000 Hz.

Examining the specifications (especially SPL curves) of guitar loudspeakers, one readily notes that the speakers would limit the response of the amplifier anyway. Add to this the limitations in most low-cost output transformers used in guitar amplifiers …. Well, you get the idea.

The preamplifier stages and phase-splitter stages of high-fidelity amplifiers don't vary a great deal from most guitar amplifiers. The differences mainly occur because of the lower cost goals that guitar amplifier manufacturers require for competitive market position. The output stages, however, differ in several important respects.

For the guitarist (even one with the more sophisticated understanding that being a jazz musician suggests) the number one priority in amplifier selection is output power. Since the earliest days, manufacturers have understood this clearly - sometimes to the disadvantage of the consumer. Not to say that we select an amplifier exclusively on that basis, generally the trend is toward smaller packaging however that suggests that volume levels may be limited, especially when playing with a drummer. So we are always pushing the envelope.

As most that are technically oriented know, the typical vacuum tube amplifier is usually overstressed in several areas, most notably plate voltage and screen grid voltage, as I've written about previously. This is intentional and simply results from the manufacturer attempting to squeeze every available watt from tubes that really don't have the quality/reliability that was typical in the last century. We don't seem to pay much of a price for this except in the areas of shorter tube life and increased distortion.

An audiophile, examining the plate curves and design graphics of a typical guitar amplifier would probably be apprehensive about the amount of distortion suggested in the design data. The audiophile would probably assume the amplifier to be virtually unlistenable. Most guitarists wouldn't share this view at all, we routinely allow higher distortion levels in our equipment and there is justification for this.

Distortion in a stereo amplifier, for example, would affect the characteristics of every single instrument in a recording playback. This might be an entire symphony orchestra, and the effect of that would be unacceptable due to an effect that we call "multi-tone intermodulation distortion". (We don't want to get into that here other than to state that multi-tone distortion is a far more serious problem than single-tone distortion and the fewer the tones, the better.)

If the playback, for example, contained only a single instrument, it's likely that the human ear wouldn't even detect the difference between a distorted signal and a perfect single-tone signal. (Differences, if detected, would probably be interpreted as "tonal".) When comparing high-fidelity amplifiers with guitar amplifiers, the same standards obviously do not apply. Most of the literature in this area supports this statement.

So at this point, the obvious question would be how much distortion is permissible for our particular amplifier ? There's no easy answer for this although many scholarly studies have been conducted on the subject. One clue might be that we seem to be happy with what the industry gives us and, as mentioned earlier, 5% distortion is the usual amount permitted during an output power measurement. (For multi-tone distortion, as would be experienced listening to orchestral music, a consensus exists that levels as low as 1% can be audible to astute listeners.)

The calculation of distortion is not particularly difficult (at least for the first two orders of harmonic distortion) but requires a lot of work, plotting things on the plate curves and extracting graphical information is sometimes difficult and not all that accurate. Distortion characteristics are of more interest to the manufacturers that produce amplifiers. Their engineers typically use computer simulations or simply make measurements on a "breadboard" amplifier.

There are a number of causes for distortion, the major ones being the non-linearity of vacuum tube performance characteristics. These are simply inherent to the devices and it's frankly amazing to me that the long-dead designers of these tubes were able to obtain the level of performance from them that was established and documented. Having said that, there are a few practical suggestions to improve tube linearity:

1. Employing "ultra-linear" output transformers with special screen grid feedback connections (expensive).

2. Selecting a tube that has very consistent plate characteristics, specifically referring to the spacing between the control grid 1 curves (the more uniform, the better).

3. Careful design of screen grid bias circuit (use a regulated voltage).

4. Utilizing the screen grid as the input rather than the control grid (screen grid draws current, control grid doesn't, so this would require an additional power amplifier to precede the output amplifier).

5. Use of negative feedback (most cost-effective technique)

Very high performance high-fidelity amplifiers were developed and produced during the decade of the 1950s, using combinations of the above techniques. These produced very high quality music reproduction that, even today, rivals solid state amplifiers. They are very costly to produce and extremely impressive to look at.

The most common - actually the ONLY - method used in guitar amplifiers to improve distortion is negative feedback. We will discuss negative feedback later, it's not a simple topic and deserves separate treatment. For our purposes now, let's just leave it at this: negative feedback uses a small amount of the output voltage of the power amplifier to pre-distort one of the input stages further back in the amplifier chain, the result is that distortion is minimized appreciably.

Illustrating the concept, here are two computer simulations showing how distortion "looks" from the viewpoint of two different measurement techniques. The first simulation first shows the output waveform (at the speaker terminal) as it would appear on an oscilloscope. Looking carefully, the bottom portion of the waveform is "flattened" compared to the top. This is exactly what our example amplifier, which had gain variations at the two extremes of the load line, would look like if measured:



This simulation shows the output waveform of the same amplifier WITH negative feedback included in the circuit, the waveform is now symmetrical:



Oscilloscope waveforms are not especially useful for measuring small amounts of distortion, so frequently an instrument called a "spectrum analyzer" is used. Very large signals can be displayed along with very small signals because the scale is logarithmic, it's calibrated in decibels (dB).

This is the same simulation as the first one, examining the output of the amplifier except using a spectrum analyzer instead of an oscilloscope. The large signal is the desired signal tone - the single audio signal at a frequency of 1 kHz that is being introduced to the amplifier input. Note that harmonics occur at levels ranging from -30 dB to -65 dB from the fundamental signal. This would be representative of an output stage with about 5% distortion:



This is the output of the amplifier in the second simulation, where negative feedback has been added to the circuit. Although it looks similar to the above spectrum, the harmonic distortion has been substantially reduced when the individual signals are examined closely. The reduction in harmonic content is 10 to 25 dB. In other words, if we use the conversions from relative power levels to dB ratios that we've learned from previous discussion, the power levels of the harmonics have been reduced by amounts ranging from factors of 10 to over 300 ! This amplifier would have distortion around 1.5%.



As we noted above, addition of negative feedback is very cost effective (the addition of one resistor, sometimes a capacitor). The improvement in performance is almost miraculous and is also predictable. Negative feedback is also useful for enhancing certain performance parameters of other circuit elements besides the output tubes - the output transformer, for example.

Negative feedback can correct most distortion problems in output stage design except those caused by defective parts or poor design. With the addition of one resistor, we've changed an amplifier with the distortion characteristics of a typical guitar amplifier to one that would be representative of a good quality vacuum tube stereo amplifier.