I have explored all sorts of variants of this basic circuit for about two years now, and have learned what I like and understood why. This is a deceptively simple circuit, except that there is subtle complexity. Often, we get caught up in the conventional way of doing things, and then spend all our time refining them to try and hide all of the problems and colorations. Sometimes we need to think outside the box. I left traditional power supply design for tube amps probably 5 or 6 years ago. There are huge threads on the sonics of various capacitors in speaker crossovers and amplifiers. Trust me, I have heard many of the top caps. The best cap is no cap. It took time to arrive at that notion. Time that was wasted auditioning top capacitors instead of thinking outside that box. As we discussed earlier in the thread, your alternatives are to either directly couple tubes, or use transformer coupling. Both have advantages in different sorts of circuits. In this circuit, transformer coupling is best because of the balance at all stages, which is the key to this amp. Think of it as constantly canceling distortion and balancing itself. But you cannot just buy off the shelf transformers from company L or H or E or whomever. For this circuit you need interstage transformers that are optimized for the loads they see and can give flat response from below 20 Hz to above 20 KHz with little or no phase shift. That is no easy task, and you have to work with a very experienced winder and it takes some prototyping and testing. Well, over a year of prototyping and testing. It is not an accident that the best of the vintage tube amps had killer transformers. So, as Lynn elegantly stated above, this circuit is totally revealing and even though you have relatively few parts in the signal path, they have to be really good parts, and some of them have to be custom designed to get the best performance. The audio circuit looks trivial, except that it isn't trivial to get that hand full of parts to work really well together.
Two years ago I would never have built a cathode biased amp, and I resisted that notion at first. Then I thought outside that box and understood what this circuit was doing and why the cathode biasing was best. I had the notion that all cathode biased amps sound slow and syrupy and deliver far less power than a fixed bias alternative. Well, in a conventional circuit that is correct, but not in this one.
The power supply is also highly optimized and we use some tricks to further isolate it from the audio circuit, and ways to isolate the input tube from the driver circuit power supply. Of course the 300b supply is separate from the driver and input supply. The filament supplies also are regulated and isolated from each other. So there is a fairly complex, yet very conservatively rated and reliable power supply that drives this subtly complex, but fairly simple circuit. If you change one thing you instantly hear it. So, in this amp there was quite a bit of tuning, again, to remove coloration. The result is a transparency I have not heard in any other amp to date, plus the ability to drive quite an array of speakers. Again, don't let the 27 watts fool you.
Is this a perfect amplifier? Of course not. There is no perfect amplifier. Again, if I were to magically create a straight wire with gain and play it for 100 folks, half of them would love it, and half would probably hate it. This amp's hallmark is utter transparency and tonal correctness. The piano is in the room with you and you can easily tell a Steinway from a Bosendorfer. I realize that most everything I have heard to date is quite colored, or if fairly neutral, lacks the resolution of this circuit built this way with these parts. This circuit is uncolored, transparent, and highly resolving, and has a boatload of driving ability. It packs serious punch. Unlike a single ended amp, where the idea is to tune the coloration inherent in the design to suit your taste, this project was about removing the coloration so the circuit could really shine. Trust me, it doesn't sound cold and clinical. It sounds like music with all over the overtones in the instruments, the inflections of the voices, etc... It is not sterile sounding at all, but rather it invites you into the music.
To each their own, but this amp is wonderful to my ear and ready for production and this thread has been an insight into our design choices, and the journey. Others make other choices and that is as it should be. I have no desire to build a 200+ watt amp to drive a very difficult speaker, but I do want to drive most speakers in most rooms.
