Class A Watts

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.

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.