Class A Watts

For me Pass Labs makes the best overwhole sound for amp's. It is better in timing and drive compared to most tube amp's. But you still have the warm musical sound. Like tube it gives a very deep and wide stage. With many tube amps I Always miss something.

In general, tube amps have output transformers- OTL amps being the exception. So tube amps generally have high impedance output because of the transformers. That means they like high impedance speakers. When the speaker impedance goes low the amperage capability of a tube amp becomes limited. That means the tubes are not able to generate the same voltage levels at lower impedances that they are able to generate with a high impedance speaker. You can short the outputs of a tube amp and not cause any damage. The voltage goes to almost zero. (Don't try that with a SS amp- it will pop because current will try to go to infinity.) Tube amps typically have power output ratings that are the same for 8 ohms and 4 ohms impedances. In contrast, SS amps are low impedance designs. They typically increase their power ratings from 8 ohms to 4 ohms impedance. The big monsters with beefy power supplies can generate gobs of amperage and so their power ratings double from 8 ohms to 4 ohms and then to 2 ohms Some even double at 1 ohm. They probably couldn't run at that power level for very long, however, without overheating. Now a tube amp would have the advantage over SS with a 16 ohm speaker. The tube amp would maintain the same power rating as at 8 ohms- because it could generate higher voltages going into the 16 ohm speaker. The SS amp, however will halve its power rating from 8 ohms to 16 ohms- just as it can double going to 4 ohms, since it's amperage capability is reduced due to the 16 ohm impedance. SS amps and 4 ohm speakers were made for each other. Sure, some tube amps work too- ideally the output transformers are matched or have taps for 4 ohm impedance.
As for Class A power- it's simply a design standard based on the bias or offset voltage/current. I don't see how a class A watt can sound louder than a class A/B watt. Almost all SS amps and tube amps for that matter run class A to some power level and switch automatically over to A/B mode once the bias limit is reached. That power level is based on the bias setting which is related to the amount of heat sinks and idle temperature that the amp has.

So tube amps generally have high impedance output because of the transformers.

Tonywinsc, this is not a correct statement. Tube amps have a high output impedance due to the high output impedance of the tube itself. You need many tubes in parallel to get the overall output impedance low (see some of Atma-sphere amps, CAT JL2, etc). The output transformer is used to transform the high(er) output impedance on the tube side to the lower output impedance on the speaker side. Hence the 8/4/2 Ohm taps on a power amp. So, output transformers save the days for tube amps - without output transformers one would not be able to drive a speaker unless one used special techniques like OTL (Atma-sphere Berning). Thanks.

Yes, of course. I was trying to keep the description more simplified, ie. the output tubes are coupled through the transformers so that the speakers see only the transformers.

If an amp is running some bias current so that the output transistors are always on but the bias current is (very) low then essentially the amp is biased into class-A. I think that they call these power amps 'sliding class-A power amps' meaning that they have some very low bias current during off-state & the bias increases proportional to the input music signal when it's present.

All transistor amps run some bias current on the output devices at idle. This is not sliding class A, this is AB. Sliding A is where the bias is dynamically changed as the input signal changes.

I was trying to keep the description more simplified, ie. the output tubes are coupled through the transformers so that the speakers see only the transformers.

The transformer transforms the speaker impedance to something that the tubes can handle. Conversely, the tubes are part of the source impedance that the speaker 'sees'.

An implication of this is that a class A tube power amplifier may well be operating in the AB region if the transformer has the wrong impedance on a particular tap, like a 4 ohm load on the 8 ohm tap.

BTW it is not correct to say

SS amps and 4 ohm speakers were made for each other.

The why of it is that all amps, tube or transistor (or class D) make more distortion into 4 ohms as opposed to 8 or 16 ohms. This distortion is audible as increased brightness and loss of detail. Put in a nutshell, if **Sound Quality** is your goal, it is best served by a speaker of 8 ohms or more. If **Sound Pressure** is your goal, there is a weak argument for 4 ohms if you have a transistor amp that can double its power (3db). Put another way, there really isn't an argument for 4 ohms in high end audio, but there might be in sound reinforcement.