40+ watts SET, cost is not a problem

the listening distance is indeed near field (6-7 feet from the plane of the speakers). However, I do understand your point of these speakers needing a lot of power. Why do you suggest Class D and not A, AB ?

@pani Most solid state amps lack the Gain Bandwidth Product to support the feedback they have. As a result, at some point in the audio band (often, only 1KHz), the feedback begins to decrease on a 6dB/octave slope, often faster as frequency increases. Distortion thus rises on a complementary curve.

This sort of thing is audible, usually as harshness (since higher ordered harmonics are unmasked in this manner) and brightness (for the same reason).

OTOH its very easy to generate enormous Gain Bandwidth Product with a class D design. That makes it possible to have feedback with no rise in distortion anywhere in the audio band, much like a zero feedback amp but with lower distortion, which results in a more musical presentation with greater detail. Some class D amps, as a result, can sound every bit as musical and involving as the best SETs, with greater transparency.

’Nearfield’ is usually about 3-4 feet FWIW. If you really are limited to that distance you might find a tube amp that can do the job. The reason the Sensitivity spec (2.83 Volts/1 meter) is used is because solid state amps can double power as impedance is halved. Being able to do that defines the amp as a Voltage source and that is what the speaker is designed for. Tube amps can behave as a Voltage source too, but do it by cutting power in half as impedance is doubled. So tubes tend to be a lot more expensive on that account. Since tubes don’t double power as impedance is halved, the Efficiency spec (1 Watt/1 meter) is more useful with tubes because it tells you at a glance if you have enough power. 83dB in my room would require 800 Watts(!) and I really don’t know of anyone making an amp that is that powerful and also sounds like real music.

Consider that if you had a speaker of 93dB you would not need to take a hit on resolution or bandwidth, but the amp needed would be a 10th of the power. So you could do the same sound pressure with 80 Watts instead of 800 in my example just above. Now its easy. If nearfield you really could use an SET (although not to its best advantage).

Right now its simply out of the question. If you want to do your friend right, really think about a different speaker.

Having heard multiple Berning amps, they ultimately sound a bit sterile to me because of the switch mode power supply.

Its highly unlikely the switch mode supply has anything to do with what you heard! When you hear differences in amplifiers, again its the distortion of that amp that is its sonic signature. The SMPS is operating at 250KHz in that design; you’re not going to hear it. IOW I get that you might not like the sound of the amp but the SMPS isn’t why it sounds that way.

I conquer with you completely. There is something so natural about the way SET present music. What it does with distortion is true and theoretical but it must be preserving something vital to make music sound so "right".

Having said that I am totally open to exploring PP amps. @atmasphere will your Novacron work well with a load like TAD CR1x ? Normally OTLs also need friendly loads. Hence I ask

@pani That speaker is not tube-friendly!! Being both 4 Ohms and sensitivity of only 86dB (meaning its efficiency of 1 Watt 1 meter is really only 83dB, which is borderline criminal), you need a lot of power! If you are only listening near field you might be alright, but if in a living room you’ll need 400 to 1000 Watts in most cases.

We discontinued the Novacron a couple of years ago- they were always limited editions. To use a set with those speakers you would need a set of ZEROs, which I think would be a good idea with any SET as well. But that won’t solve the power issue!

So I really think you need to rethink this, starting with a different speaker if you are committed to lower powered tube amps! FWIW, our class D might be alright on that speaker (I have reservations since it only makes 250 Watts into 4 Ohms at clipping); it has a distortion profile that is very similar to that of SETs although its quite a bit lower distortion overall.

And that brings me to something else you might want to understand:

The distortion characteristic of any amplifier is also the sonic quality of that amp. Put another way, distortion is far more audible than most people realize. When you compare amps (unless there is a frequency response error that might also accompany an amp of high output impedance) the differences you hear are the differences in distortion.

As a result it does not matter what the technology of the amplifier actually is; what matters a lot is how the amplifier makes distortion. The reason SETs sound the way they do is because of a quadratic non-linearity which results in a pretty prodigious 2nd harmonic, followed by a 3rd which is considerably less but also prodigious with respect to the higher orders, which, if the amp is designed properly, will fall off on an exponential curve as the order of the harmonic is increased. The 2nd and 3rd are able to mask these harmonics, resulting in a smooth sounding amp.

I hope you can see here that we really can measure what you hear and that it really is all engineering; the real issue is whether the designer of an amplifier understood this fact.

If you have a PP zero feedback tube amp that is fully balanced and differential from input to output, the even orders cancel, leaving the 3rd as the dominant harmonic (which is treated by the ear much the same as the 2nd), still able to mask higher orders. Because the even orders are cancelled, there is less distortion compounded from stage to stage in the amp so distortion is inherently lower. But you still have the exponential decay of higher orders, only now based on a cubic exponent, so they fall off at a faster rate than seen in an SET. That is why such amps are even more relaxed and also more transparent, since distortion obscures detail.

If you have a PP amp that uses single-ended input circuitry (like most PP tube amps) you combine the cubic and quadratic non-linearities, which tends to augment the 5th harmonic (and this has nothing to do with poorly applied feedback, which I touched on in an earlier post). I think this is (and understandably so) why the SET crowd disdains PP. But not all PP amps are built that way!

This is the tip of the iceberg!

In conclusion I really think what you are trying to do is a Sisyphean task. An SET simply will not work and all PP tube amps will be on shaky ground as well.

I know this isn’t what you were asking for, but your best bet here really is a class D, although you should keep in mind that class D amps vary in sound from one to another more than tube amps do- so the idea that you’ve heard one you’ve heard them all is false. The reason I say a class D amp is your best bet with that speaker is because you need the power, and some class D amps are smoother than any other kind of solid state and can be just as involving and spacious as the best SETs. I mentioned part of why this can be so in an earlier post. If you wish elucidation I can provide it.

I have owned a 300b SET for 15 years and simply love it. However any DHT can be utilized in a SET amplifier. 2A3, 45, 211,805,GM70 etc. SET is not limited to300b.

@charles1dad 300b is about as powerful as you can get and still claim hifi bandwidth. The thing is, if the amp doesn’t employ feedback, you’ll get phase shift down to about 10th the upper cutoff frequency (-3dB point) and 10x the lower cutoff. This is filter theory.

The ear is terrible hearing phase by itself, but over a spectrum it can interpret it as tonality.

Phase shift in the bass due to a cutoff frequency not low enough is audible as a lack of bass impact. Phase shift in the highs due to a lack of HF bandwidth is heard as a darkness. So you need bandwidth well beyond the audio band to really get this part right; with no feedback ideally 2Hz to 100KHz or better.

@atmasphere @charles1dad @jond and others, it seems people are not warming up to the idea of SET here. And also there seems to be very few seriously good tried and tested choices beyond 25 watts. So what are some really good options for push-pull tube amps operating in class A with zero negative feedback?

@pani There is nothing wrong with feedback if its properly applied! The problem is, it usually isn’t. As a hint you can’t apply it to the cathode of an input tube since that tube will distort the feedback signal. That will cause IMD and higher ordered harmonic generation, both of which are not musical. This is one of the reasons feedback has a bad rap, but its not feedback’s fault so much as poor execution.

An advantage of zero feedback though is you get a ruler flat distortion vs frequency curve. This is important; if there is a rise in distortion at higher frequencies the ear will interpret that has harshness and brightness because the harmonics will not be masked (as they usually are in an SET). I know a PP amp that is zero feedback isn’t that hard to find. Take a look at the ’300b lovers’ thread on this site.

If you do feedback correctly you can get a ruler flat distortion vs frequency curve just like a zero feedback amp, but with lower distortion overall; just as smooth as any zero feedback amp, just as laid back, but with greater resolution since distortion obscures detail.

@pani A 40 Watt SET will have real problems making bandwidth. That is why the smaller SETs sound better as @larryi  pointed out.

Do yourself a favor- if you want to get the most out of your amplifier dollar, its not going to happen with an SET of that sort of power, and throwing more money at it won't overcome the problems you have with the larger output transformers needed to make that sort of power (and output transformer designer can tell you this). Either get a speaker that can be driven easily by an SET of much less power, or get a PP amp that can do the job properly.

There is no reason a PP amp can't be every bit as musical and involving as the best SETs out there; power notwithstanding. Its all a matter of good engineering and execution.

Here is something else to think about: when SETs are compared to PP its usually apples and oranges. The PP amp likely isn't class A, uses pentodes and probably also has feedback. How about level the playing field? How about a PP amp that uses the same power tubes as the SET, is also class A and zero feedback? When you level the playing field in this manner, you find out real quick that SETs just really aren't that good, while the power tubes themselves really are. Do you see what I'm saying?