In hi-fi amplifiers, one of the things many designers aim to do is to reduce distortion. Yet to do this almost inevitably involves the use of negative feedback, which, while it does indeed lower the amount of distortion in the amplifier, at the same time has a rather nasty tendency to make the amplifier sound “dead,” or sound less like an actual live performance.
To be sure, sometimes feedback really does provide an improvement in overall sound quality, such as in OTL tube amplifiers where without it the output impedance and distortion levels are usually much too high. But it is generally frowned upon, and often with good reason; after all, what matters isn’t the numbers, but how well the amplifier actually sounds — and the two don’t always directly correlate.
The good news is that there are ways to improve the quality of the sound that the ear hears as well as the distortion numbers without compromising by using feedback.
One way to lower distortion without using feedback is to increase the tube’s plate load (PL) resistance. The reason this works is because, in a triode, the primary characteristic that changes as it amplifies is the plate resistance (the internal impedance of the tube, shown as RP) — not the tube’s amplification factor (u).
The gain of a given tube stage is determined by the formula:
Gain = (u * PL)/(PL + RP)
It follows that either a variant in a tube’s amplification factor or plate resistance will cause a change in gain and hence distortion.
Now, as stated before, the amplification factor in a triode is pretty much steady, while the plate resistance changes quite a bit. However, as seen in the formula, if a high value of plate load is used — a value significantly higher than the plate resistance — the gain of a tube stage will remain stable despite a change in the tube’s plate resistance. In effect, the high value of plate load swamps the changes in plate resistance.
Try the formula — it really works. With a set change in plate resistance, you will see that as you increase the plate load, the changes in gain decreases. Values of plate load at least twice as high as the plate resistance are recommended. Experientially speaking, values of load around five to six times the plate resistance are ideal for triode power amplifiers optimized for lowest distortion operation — just note, your mileage may vary.
There is of course a downside to this, which is pretty much only a problem in power amplifier stages: loss of output power. In your tube manual, the values of plate loads shown are optimized for maximum output power obtainable without distortion levels skyrocketing. Therefore, by increasing the plate load, the power output will decrease.
The Tung-Sol 2A3 datasheet has a graph which shows this nicely. The distortion can be cut in about half by doubling the plate load from the standard 2.5K value used with this tube, but the power output will drop to 2.5W from about 3.5W. It’s the price you pay for lower distortion.
One final thing. The graphs of tube plate curves are very valuable for determining distortion, and can be used to pick a very linear operating point. Simply look for places where the distances between the various curves are symmetrical. So for instance, if a decrease in negative grid voltage of 10V causes an increase in plate current of 10mA, in a perfectly linear part of the curve, an increase in grid voltage of 10V should cause a decrease of 10mA plate current. Searching for these linear parts of the tube’s operating points can be of invaluable service.