more reliable amp: tube or solid state class A

+++ It always amazes me when someone using this site says they've been using the same equipment for more than two years or so. +++

What an strange comment. I have equipment that I will listen to till I go to the big sound-room in the sky. Incidentally, that includes a pair of Meadowlarks; you seem to be hanging onto a pair also.

BTW, Meadowlarks were designed to be used with vacuum tube amplifiers.

Regards
Paul

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.

Hi David, Your comment that the 6AS7 is not that linear is not really true. The curve has a lot in common with a 300b and also 2A3s; in that regard the 6AS7G is in good company. On that point the idea that the amp is class A due to the non-linearity of the tube does not hold up. BTW if you study the RCA documents that they released on the tube back in the early 50s, RCA called the tube an audio amplifier *and* VR (Voltage Regulation) tube. FWIW for best VR operation linearity in the pass element (in this case the 6AS7G) tube is important!

While power supply stability is important, I don't think I would say that is why you do class A. IMO, you do it because of the increased linearity of the device. Particularly in push-pull, very low zero feedback distortion levels are possible. Achieving low distortion without feedback allows for lower amounts of higher-ordered harmonics, which are loudness cues for the human ear. Sort of a have your cake and eat it too thing.

Our power supply voltages are not as high as you describe, and it is a fact that the tubes operate class A2, which is something that I am happy to point out should anyone ask; like, right now :)

In A2 there is substantial grid current during part of the waveform, similar to class AB2. Our driver circuit is designed to handle the current, similar to the way Fisher did their class A2 amp back in the 50s. The 6AS7G is remarkably linear in the A2 window, like a lot of power triodes are (we built a 300b OTL once just to see if there was any advantage; the 300b has a similar A2 window BTW).

If the bias and B+ points were a bit different (IOW if the tubes went into cutoff before clipping; right now they cut off only *after* the amp clips), the amp would be class AB2, not A2.

I am if the opinion that class AB is harder to design for and get right since the driver waveforms have to be larger amplitude and the driver power supply has to be more stable. If you are trying to set things up without feedback, which is what I would do, class AB gives you less opportunity for distortion cancellation so you really have to have your ducks in a row to make it work.

Mr Berning and Mr Karsten (two of my favorite designers... FWIW) thanks for getting into the discussion. I have questions about some of the same issues and your input is helpful.

That's one of the (few) good things about AudiogoN, getting first-hand, good information from people who know what they're talking about...

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This is why I come back here so often. Geesh, I'm not even a tube guy, but to get a chance to listen in on a discsussion between Mr. Berning and Mr. Karsten is a real treat indeed! Thank you gentlemen.