Class A Watts

06-16-13: Csontos
However, there are A/B amps with no switching distortion and much quicker than lots of class A amps....

Csontos, such an amplifier is, then, not class-ABl it's a class-A amp with some very low off-state bias current. I believe this amp is in the fuzzy region of amplifier classification - what I mean is that, traditionally, pure class-A power amps have their rated output current running thru their output stage whether there is a music signal or not hence they get very (, very) hot just idling. And, traditionally, class-AB amps do not have any bias current running thru the output stage meaning that only one output transistor operates at any given time & the other is off as in totally shut-off. This is what gives the cross-over distortion.
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.

06-17-13: Phd
A watt is a watt unless you are comparing ss watts to tubes watts.

Phd, a watt is a watt is a watt, no matter what! what say?
that includes tube watts.
From Ralph's many posts on the 'voltage paradigm' vs. the 'power paradigm' I'm made to understand that ss amps output a constant voltage & modulate output current as the speaker impedance varies. Tube power amps, OTOH, output a constant power & modulate both voltage & current as the speaker load varies (while keeping the product of voltage times current = output power constant). So, you "feel" that tube watts go a longer way than ss watts.
But 1 watt is 1 watt & will always be so, whether tube or ss.

06-17-13: Phd
Bombaywalla, you know what is, watt for watt tube amps appear to have more power and it could be due to the fact that tube amps clip more softly. Are you buying this?
Phd (Threads | Answers | This Thread)

Phd, I'm struggling to. SS amps can also be designed in such a way that they clip softly just like tube amps. It's all in the implementation details.
Tubes (no matter how old they are) are still (today) a linear amplification device. Tube amps can be designed by using Ohm's Law. OTOH, ss amps are based on semiconductor devices & one needs to have a basic understanding of semiconductor device physics to design with them + ss devices (BJTs, MOSFETs, JFETs, IGBTs, etc) are all exponential/square law devices i.e. not linear amplification devices like tubes. Hence the ss device distortions patterns are quite different from tube devices. I think that is why you feel that tubes clip softly while ss amps do not.

06-17-13: Lowrider57
Bombaywalla... for clarification, are you explaining what some manufacturers such as Plinius refer to as Class A/AB?
thanks.

yes, I believe so altho' Plinius is not a good example. All the Plinius amps I know of actually have a switch that can switch the amp into class-AB & into class-A. In fact, these amps have thermal protection that can be set to 15 min/30 min/60 min where the amp falls into class-AB mode if un-used in class-A mode. So, in Plinius' case, they are actually changing the bias current with deliberate user input (toggle that switch). The sliding class-A amps that I was talking about do the class-A/AB transition automatically with no user input.

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.