more reliable amp: tube or solid state class A

Hi Ralph-
You are technically correct according to most published definitions of class A, which usually define class A as being an amplifier in which output devices conduct for the entire cycle. However, I should point out that the definition of class A is a very loose one and can easily be met without meeting the true intent of class A. One of the most important attributes of the class A amplifier is that the output devices are biased so that power drawn from the power supply is constant regardless of signal. This attribute is an important one if one is to obtain the full sonic benefits of class A.

Now let's look at your 60 watt amp. I believe you use four type 6AS7 dual triodes for the push and four more for the pull. Each triode is rated for 13 watts dissipation, and you have eight of these in parallel for the push. I believe you are using about 160 volts for each power supply, so 160 volts is applied across each of eight triodes. In order to not exceed the maximum tube ratings, your idle current can not exceed 0.65 A for the combined bank of tubes. Now, if one were to assume linear amplifying devices such that one group of triodes turned off at the same rate that the other turned on, an idle current of 1.95 A is required to meet the peak speaker current requirement of 3.9 A into 8 ohms. This would be 120 watts peak power into 8 ohms (peak amps squared times load resistance) or 60 watts RMS into 8 ohms. Therefore, you are operating at a bias current of only one third of what would be required to maintain a constant power flow from the power supply.

Now how is it possible to meet the commonly published definition of class A with your amplifier? The reason it is possible, is because the definition does not address device or circuit linearity, all it says is that no output device can be turned off for the entire cycle.

The 6AS7 was never designed as an audio amplifier tube but rather a series pass tube for voltage regulators. The 6AS7 is a very non-linear tube and is difficult to actually turn off completely. When these tubes are used in a push-pull application, such as your amplifier, one bank of tubes turns on much more than the other turns off, so much so that it never turns off. Voila! class A!

To me, your amp meets the intent of class AB, and this is good. In fact, the point of my previous post was to criticize the idea that class A was required, especially for tube amps. Your amp is an excellent demonstration of why tubes can work so well in class AB in that they can have a very soft or gentle turn-off, and in the extreme, no turn-off at all (meeting the loose definition of class A).

I should point out that there are a number of SS amp manufacturers that use tailored bias schemes to cause the output transistors to not turn off, and they like to call their amps class A. I have seen some of these at shows, and you put your hand on them and they are barely warm. The problem with the definition of class A is that it is so loose that these amps indeed meet the definition. So...buyer be where. These amps certainly in general do not sound as good as a true class A SS amps because they still have abrupt discontinuities in the current through the output transistors, and the power supply current is highly modulated by the audio signal. Furthermore, if the user of my Berning ZH270 wants class A operation, it can be obtained with no changes to the amplifier; all one has to do is to find a set of poorly made output tubes that don't turn off completely to meet the definition of class A.

In summary, the point of this post is that the buyer should pay less attention to amp names, specs and classifications and listen with an open mind. In particular, the definition of class A is so loose that anything can be made to meet it, and I feel that the soft turn-off characteristics of tubes makes these devices ideal for practical amplifiers that do not have to be excessively wasteful of energy.

Amp reliability is determined by the level of conservative design, and this applies to tube and SS. The reason that SS class A amps tend to be more reliable than tube class A amps is that a SS amp will blow up immediately if it is not designed with adequate margin. With tubes, you can get away with marginal design for a few months before failure. Designers of SS amps HAVE to build in more safety margin. Class A tubes amps can be built to very high reliability levels, but they have to be designed very conservatively. I do not know of any class A tube amps in this category. For example, a 6L6 tube is rated at a plate dissipation of around 30-35 watts. They are typically run at this level. If I build a class A amp out of these tubes, I will run them at a dissipation of no more than 10-12 watts. They will run many years under these conditions. The amp manufacturer needs to use six of these tubes (per channel) to make a 30-35 watt amp reliable in a tetrode mode transformer or ZOTL coupled amplifier. More are required in an ultralinear or triode mode amplifier. Many more would be needed for a traditional OTL amp, assuming that these tubes were appropriate for that type of amplifier (they are not).

On the other hand, it is of questionable merit to demand a class A tube amp, as tubes behave much better than SS devices in a class AB application. SS devices have much more abrupt cutoff characteristics than tubes, and as a result, they inject high-order harmonics in the crossover region. Tubes have a very soft turn-off and as a result, they don't inject these harmonics to the same degree. The gain in going from class AB to class A in the SS amp is much greater than it is in the tube amp. An fact, the class A tube amp makes little sense, unless it is a very low power amp. The idea of making a hot amp in general does not make sense to me in unless it is strictly a winter amp and you can supplement the work that your furnace has to do. To use that amp when you need to apply air conditioning to cool it is stupid. How does the noise of the blower affect your room noise level and your attempt to achieve good sound? We all also need to think more responsibly when it comes to our energy use. We have limited resources and we may be doing damage to our climate with excessive energy use.