Why do tube amps often subjectively sound more powerful than SS ?

I had demonstrated to me by my friend Murray Zeligman and David Berning that full loop negative feed back was deleterious to linearity(a Berning amp with variable feedback and as the feedback was reduced to zero what I heard sounded better to me).

@dynamiclinearity  If the feedback isn't set up right you will get exactly this sort of result. One of the problems is the feedback is sent to the cathode of the input tube. The tube isn't linear of course, so the feedback signal is distorted. This causes additional higher ordered harmonics. Norman Crowhurst wrote about this problem nearly 70 years ago but its rare that anyone has done anything about it. 

The solution to that problem of course (one we've used for decades) is to apply the feedback to the grid of the tube rather than the cathode, in a manner similar to how its done with opamps. The feedback is thus mixed with the audio in a resistive divider network which is far more linear than any tube or transistor. This reduces the harmonic and intermodulation distortion generation you otherwise get. Of course you have to sort out how to get the phase right but if an output transformer is involved its easy.

Actually you don't need a degree to design tube amps.

@dynamiclinearity That is certainly true. But it helps, especially if the amp in question has a feedback loop. Designing a good feedback loop is a bit less trivial if you want to get it right.

My point was there's been a resurgence of SETs since the early 1990s, most of which don't use feedback. They tend to have a high output impedance and so speakers that expect the amp to behave like a Voltage source (which is most speakers made) don't work right with them. Hence along with SETs we have more horn and open baffle designs meant to work with low power tube amps with high output impedance.

If Stereophile was measuring such an amp it wouldn't measure well by their standards but might sound pretty decent.

Some tube amps I would expect to have a higher damping factor are those made by Roger Modjesky.

Futterman made some OTLs with quite high feedback and had damping factors as high as 40:1. You can put more feedback on an OTL since you don't have the poles created by the output transformer; you can have a greater phase margin. Khron Hite made some laboratory amps (UA-101) in the early 1960s that had as much as 80dB of feedback (and hence had very sophisticated feedback design) which had a damping factor of 100:1.

Then why do almost all the damping factors measured by Streophile fall below 10

@dynamiclinearity I don't read Stereophile so I've no idea if this is actually true and if so, what amps to which you are referring.

It might be a better question to ask who the designer was and what was their intention. Did the designer have a degree? FWIW, most of the tube amps designed in the old days were designed by actual engineers. That's a lot less common these days. Not saying you need a degree to know what you're doing but it helps!

Some speakers, for example some open baffle designs, really don't do well if the amp has a high damping factor. Nelson Pass demonstrated this about 15 years ago a RMAF using a set of open baffle speakers and a solid state amp he built that was constant current rather than constant Voltage, so its damping factor was fractional since the amp's output impedance was about 50-60 Ohms. It sounded just fine and made good bass despite the small size of the OB speakers. 

So intention is a big variable here and Stereophile does not recognize that certain speakers need a higher output impedance from the amp to sound right. They just have their standard simulated speaker load and that's that.

Basil Poledouris’ incredibly dynamic score for Conan the barbarian.

Especially the opening track The Anvil Of Crom.

@prof I really like that LP. I think Poledouris did a great job on that.

@dynamiclinearity I'm going with 'a fair amount'.

The venerable Dyanco ST70 has a damping factor of about 15. This does not change with a 4 Ohm load since you'd be using the 4 Ohm taps. It would be cut in half if you ran a 4 Ohm load on the 8 Ohm tap.

ElectroVoice made a number of tube amps (such as the A30) that had a damping factor of 15 back in the 1950s. It was EV and MacIntosh who came up with the idea of the amplifier being a Voltage source and the speakers being Voltage driven; this to enhance plug and play.

Prior to that speakers had adjustments on the back (midrange and tweeter level controls) to allow the speaker to be adjusted to the unknown output impedance of the amp.

The Mac MC275 had about 15:1. The Marantz 8b amplifier had a damping factor of 16:1.  The famous Fisher SA100 was 20:1.

FWIW no speaker made needs a damping factor of over 20.

Tube amps also tend to boost the mid bass and mid range (from about 1 kHz to 2 kHz or ao) due to their low damping factor interacting with speaker impedance.

@dynamiclinearity This is only true if the amp is unable to act as a Voltage source on the speaker in question. Many tube amps are perfectly capable of such a thing. You only need about 15dB of feedback to achieve that in many cases.

In my experience, tubes tend to highlight the lower part of the spectrum, where s/s favors the upper.

@stringreen That is on account of how the amps in question make distortion. Once you understand that, its then possible to design a solid state amp (including class D) that doesn’t sound bright.

It's not really about loudness, it just feels more powerful at the same volume.

@inna, Mapman has it right. The reason the tube amps sound like they have more power is because of how they make distortion when approaching full power. Unless its an SET, with most tube amps the higher ordered harmonics start showing up in greater amounts near overload. This interacts with the ear which imparts the impression that its louder- more powerful. This has given rise to the false notion that tube power is more powerful than solid state.

But like @mapman pointed out, if you have a sound pressure level meter, you'll find that its not louder- and in fact might be quieter.

I mentioned the caveat of SETs- they take this to an extreme by producing those higher ordered harmonics at a much lower power level- perhaps only 20-25% of full power, which is why many people talk about their 'dynamics' (since power is used the most on transients). But what they are really talking about and also in this case is distortion and how it interacts with the human ear (the ear uses higher ordered harmonics to gauge sound pressure).

Tubes make it an enjoyable experience since there's plenty of second order harmonics to go around too.