40+ watts SET, cost is not a problem

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.

@atmasphere

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.

On technical grounds I won’t even try to dispute what you stated. I can only rely upon what I have heard. I’m very familiar with in particular two PSET 845 amplifiers (50 watts for each pair) that could suffer the bandwidth limitations you mentioned. I can genuinely say however that both of them sound superb and so natural reproducing music.

I am referring to the Absolare Passion 845 mono blocks and the Viva Audio Aurora 845 mono blocks. I don’t know how they conquer/manage the potential bandwidth limitations. I can attest that listening to music, Both are stunningly terrific! No doubt their respective designers/engineers could explain how they accomplished this.

Charles

@pani 

I own the Coincident Frankenstein 300b SET mono blocks. They also build a 211 push pull  (Class A zero NFB) mono block named the Dragon. It’s 75 watts per channel. By all accounts it is fabulous sounding.

Charles

My friend just finished some 52 watt single ended triodes he will be selling 

@charles1dad 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