NOS Tubes--Are they worth it? or accurate?

Pls 1, since you have engineeering background, do you believe that it is not possible to design an accurate tube amp or preamp? If, for example, you look at bench measurement frequency response on a Sonic Frontiers Power 2 or 3, you will find almost a dead flat response. You don't see that very often in tube amp reviews, I know. However, again, into an actual load, who knows? But doesn't that apply to transistor designs as well?

Regarding the SF amps, from my experience and everything I have read, listeners still find there are deviations from flat in these amps. Maybe I'm answering my own question here. That we don't know how to design a truly accurate piece of equipment.

Be that as it may, it still seems that pieces that show gross frequency response deviations show these dips and peaks in their sonics as well. So, what do measurements tell us? Something and nothing both.

You raise some good points. I've got a bunch of client stuff over the next few days so it will take me some time to get my thoughts together for a more complete answer.

In the mean time a quick response. I am fairly fanatical about flat frequency response and correct phase/impulse response. I own Dunlavy SC-V's and use wall treatment and a Sigtech Digital DSP to get very flat reponse.

The Speaker/room combo is usually never anywhere close to flat. Almost every amp is virtually ruler flat compared to that or to a cartridge. If you care about accurate frequency and phase reponse the speaker/room combo is where to start.

As far as measurements on electronics, it gets interesting when the steady state sine wave measurements are replaced with impulse measurements.

In theory a simple triode amp in a well designed circuit should be linear over its designed operating range. In practice it never is.

For example, the instantaneous whap of a bass drum may drop the voltage on the B+ rails that powers the tube so severely that for the instant the amp is operating nowhere near in spec. This can be detected with a 'scope and heard as a congested sound.

Some tubes are very microphonic, that is they vibrate when disturbed. In a guitar amp, this creates a desired sound that isn't "accurate". Loud music may cause vibrations in the audio amp coloring the sound.

These are just two of the dozens of these readily known departures from a completely "accurate" amp. All real world amps have these faults to some degree. This doesn't even count all of the depatures from linearity that happen through the electrical interaction of the speakers, crossovers and the amplifier playing real music.

Some faults, like even order harmonics in a SET, can be perceived as euphonic. There is no accurate mapping from the engineering circuitry to auditorily perceived accuracy. Our ears are non linear transducers. Evolution has tuned then to be amazingly sensitive to some signals and insensitive to others.

The reason "Reference" models cost so much is that every component and build decision is (should be) made around minimizing every possible fault with the hope that then the amp will be perceived as accurate. (Plus they are usually hand built in single digit qualtities).

At what point you find the sound acceptable becomes a matter of taste. Hence my comments on NOS. Your ears will detect sonic signatures from virually any piece of tube equipment. A designer who listens only to small jazz ensembles is of no use to be personally because I listen to large scale orchestral music.

More later

Good point about the designer's musical tastes. Designers should really become intimately knowledgeable of the sound of a full orchestra as well as a jazz band, a jazz ensemble, vocalists, choirs, etc.

I guess those who think they have achieved 'accuracy' are fooling themselves as our technology and knowledge is far too limited to accomplish this.

Saxo,
It sounds like you are trying to talk yourself out of upgrading the tubes. Stick with what you have if you are happy. If you find you want a little more (as most of us do) then I highly recommend upgrading the pre-amp (small signal) tubes first. It does take some research. Different tube types, different manufacturers, different amps. My pre-amp only has 4 tubes so I replaced the entire quad with a matched set from a reputable dealer here on A'gon. There was a dramatic change in sound. All the good things you read about NOS tubes. Try to find ones which are matched, tested for noise and microphonics.

I'm sure that amp designers would use Mullards, Amperex or whatever NOS style tubes if they were currently being produced. I think that MF bought up the world supply of Nu-Vista tubes so they could make a limited production run. They simply can't sell a product where the tubes are in increasingly short supply.

Everyone has very good points. I would like to add that I have NOS tubes in my ARC gear but after much research and confusion, bought them from Kevin Deal at Upscale Audio.
He knows what works and sounds best in many brands and is very good to do business with. I bought some Amperex
off ebay and never received them, so beware and buy off a reputable dealer. The tubes are worth every penny and for the improvement in sound, a steal. IMO if it sounds good, it is good!