Your journey with lower-watt tube amps -- Can a kit be good enough?

Nice blog post on SETs recently, here.

A few excerpts. It’s not a long post and worth a read:

"The SET’s very existence calls into question fundamental beliefs and assumptions we routinely make about technical performance, sound quality, and the correlation between them. These amplifiers expose a crack in the edifice of audio engineering theory that is based on the conviction that an amplifier can be judged by its technical specifications or measured performance....

This paradox arises because the technical measurements that attempt to quantify amplifier sound are simply inadequate and incomplete....Predicting an amplifier’s sound quality or judging it to be good or bad based on existing criteria is like looking at a few still images from a movie and then attempting to discern from those static photos the movie’s plot, characterizations, dramatic arc, and meaning.

The SET exposes the fact that certain aspects of amplifier performance are not quantified by the traditional measurement arsenal....

Some will suggest that listeners are merely responding to the SET’s euphonic distortion—that the SET sounds good because of its distortion rather than despite it. There’s no question that the largely second-harmonic distortion component of an SET is much more sonically benign than the upper-order distortion components of Class AB solid-state amplifiers. But a first-rate SET amplifier’s magical qualities go far beyond this simplistic interpretation. The SET’s resolution of inner detail that, singularly, conjures up a strikingly vivid picture of the instrument creating the sound is certainly not merely a euphonic second-harmonic distortion artifact....

This essay is neither a renunciation of all amplifiers other than SETs nor an evangelical campaign for the world to embrace the single-ended-triode amplifier. SETs are limited in the loudspeakers they can drive, exhibit other practical drawbacks, and are certainly not for every listener...."

https://www.theabsolutesound.com/articles/the-single-ended-triode-paradox/

I've long agreed with Harley after having been through virtually all classes of amplifiers, both tube and SS. Virtually certain most of those amps measured better than the SET's I've owned, yet none provided the immediacy, the resolution, transparency, and the micro dynamics of my SET's.

I posit at least part of this quandary is a function of low parts count in SET. SET doesn't have to negotiate multiples of caps, resistors, transistors, what have you, and if point to point wired no circuit board. I can hear sound quality differences with a changing out a single resistor, cap or hook up wire, this goes to show how transparent these amps are. I used to have to change out multiples of these items in other amps to exact a change in sound quality. This suggests a signal passing through a less circuitous route is less changed/affected/contaminated, in other words more pure to the source.

 Power supplies of all vintage tube amplifiers from 50-60x look so miserable with such small capacitors.  It is especially true for stereo amplifiers when PS is used for both channels in parallel without any separation. I can say for sure such a PS will not work for a SET amplifier without feedback even if it is just 2 watt power.   

How does it work in all these vintage amplifiers? Does feedback make them less sensitive to PS or the reason is the push-pull topology?

@alexberger  Feedback improves power supply rejection and crosstalk; ideally anything that isn't the signal. For example, in an LP mastering situation, the feedback winding on the cutter head is essential to allow channel separation. The stereo LP would not exist without it. 

Such a power supply as you described above will work fine in any SET that is running class A1 on the power tube, since at any signal level current draw is constant; IOW from idle right to full power. Consequently this statement:

The issue with small capacitors is not the 120Hz noise from AC , but voltage stability during playing of load and complex music.

-is false.

Boosted capacitance does help with a stereo amp or even a mono amp if the amp is push pull and operating class A2 or any form of class AB (IOW where current draw varies with power), so as to reduce intermodulation.

In all cases, the proof of the pudding is to put an oscilloscope probe on the power supply rail and run the amp up to full power at a variety of frequencies to see what sort of noise is present in the power supply. Since the output circuit isn't linear, power supply noise can intermodulate with the audio signal in the output section.

@hilde45 The article you quoted was true back in the 1980s but is not true now. The real issue isn't that we can't measure it, its knowing what the measurements are telling us. That knowledge is in short supply! I've been explaining exactly why SETs sound the way they do. Again, in a nutshell: They shine when not presented with bass or asked to make much power.  I've already explained why they sound good when they do so.

Harley's mistake was simply not asking amplifier designers why SETs sound the way they do. But if he had done so he might not have had an article to write I was online pointing this out when this article originally published a few years back.

I posit at least part of this quandary is a function of low parts count in SET. SET doesn't have to negotiate multiples of caps, resistors, transistors, what have you, and if point to point wired no circuit board. I can hear sound quality differences with a changing out a single resistor, cap or hook up wire, this goes to show how transparent these amps are. I used to have to change out multiples of these items in other amps to exact a change in sound quality. This suggests a signal passing through a less circuitous route is less changed/affected/contaminated, in other words more pure to the source.

@sns This statement comes to a false conclusion. The reason you hear differences due to components is not because SETs are particularly transparent (due to their having the highest distortion of any kind of amp made, and because distortion tends to obscure detail, its arguable they are the least detailed amps made), its because they lack feedback.

Feedback helps the amp reject that which is not the signal. Artifacts from wire, resistors, capacitors and transformers are not the signal, as well as noise from poor grounding, layout problems, line Voltage and so on. So when there is no feedback, everything (and I mean everything) makes a difference.

The problem of course is whether or not the feedback is applied properly and we've all heard amps where it wasn't. I've no doubt that many might prefer an SET over such amps! But its usually not due to detail- its (IME, IMO) due to amps with poorly applied feedback not sounding particularly musical on account of harshness or the like.

Thanks @atmasphere 

I would love to hear what Harley would say in response.

"@atmasphere Feedback helps the amp reject that which is not the signal. Artifacts from wire, resistors, capacitors and transformers are not the signal, as well as noise from poor grounding, layout problems, line Voltage and so on. "

Ralph can you explain in a bit more detailed manner [my own ignorance] how "feedback" sort of identifies, isolates, and/or removes these other things that are "not" the signal. A local 50-year tech now retired has debated this some about how some amp designers do and don't leverage feedback for the circuit, or in the right ways.  He actually gets somewhat aggravated in conversation about this topic, lol.