tube Watts vs solid state Watts

Duke, excellent answer. Clipping behavior is the reason that people perceive tube amps to be more powerful even when the wattage is the same.

This is also the reason that electric guitar players love tube amps. Many of the most memorable "sounds" we hear from the great guitar players are due to the way they drive various gain stages of their tube amps into intentional distortion. Do the same thing with a transistor amp and it typically just sounds nasty.

Just for full disclosure, I current use one of the solid state class T amps. If you stay out of clipping the difference between solid state and tubes is much more subtle.

The limiting factor IS wattage. Tube amps have a high output impedance and low current which means their power is limited to the output tube plate dissipation. A transformer's needed to reflect the low speaker impedance and this limits the amplifier's power output. When impedance is high, the current is lower and within the amp's power rating (but will put out less power), as the impedance drops the amp clips. Softly, it's agreed.

A solid state amp has a low output impedance and higher current which in turn requires higher capacitance and, more important, a power transformer rated higher than the amp's output power. So SS amps usually have more in reserve and put out more juice as speaker impedance drops. Maybe a 20 wpc amp at 8 ohms can put out 30 or 40 wpc at 4 ohms. Any way you slice it, this is twice the power at that given load. And the load is the defining part. You cannot say they're the same or that a tube amp is more powerful unless you consider the same range of impedances - an area where tubes generally cannot compete.

BTW 20 watt Class A amps consume more power than 20 watt Class B amps if both are rated the same - they are not more powerful watt for watt. They're just power pigs; a 20 watt Class A amp requires a 100 watt minimum power supply whereas a Class B may require a 70 watt supply.

Perceived loudness and amplifier wattage is not the same thing. Agreed a watt is a watt, no matter what (excuse the pun).

On a thread some time ago (I am to lazy to search now) a couple of posters stated that even if the fundamental remains the same, humans will perceive an increase in volume when the even ordered harmonics increase in db. So when a tube amp clips in a benign fashion, additional gain will allow the harmonics to increase in db, allowing the listener to hear an increase in volume/loudness.

I use a 12wpc SE tube amp and it most certainly plays louder than my 25wpc chip or 35wpc transistor amp. So while it is technically a watt is measured the same no matter what amplifier topology we talk about, for a specific rating a tube amp will deliver substantially more perceived loudness than a transistor amplifier.

Regards
Paul

And what about pure class A solid state Watts and class B Watts? Many audiophiles (who are often non techies) say that a 50W pure class A solid state amp is comparable to a let's say 200W class B amp. Is this bullshit? Why do people give that kind of deceiving comments?

Chris

When comparing two amps, I think the power rating is the power rating - no matter whether Class A or B as long as you are comparing at the same THD. Perhaps the discrepancy is due to type of distortion as mentioned above because Class A and Class B definitely have different distortion characterisitics. But I also feel (and this applies to tube amps too) that tonality can play a role. I often hear that such-and-such low-power tube amp has better bass than some big SS amps does and that gives the impression of more power.

Another thing I have heard a few times now is that tubes are able to produce a huge amount of power for a very short amount of time - much moreso than SS amps can. I am not sure about the validity of this claim but from a signal level conduction resistance point of view, it makes sense since you are comparing vacuum with "sand."

But when you are dealing with human perception, everything gets cloudy. Everyone is entitled to their opinion so take what you hear and read with a grain of salt but don't get upset by it. :)

Arthur