300B Tube Amps with 15/16 Watts Per Channel?

@vthokie83 Just curious.  What speakers is your friend driving with the Blackbird 300b amps?  Just another data point!

I was asking him what 300B amp (preferably mono) he would recommend. He came up with this and said it is easy to mod it to make it sound even better.

@yyzsantabarbara A nice move that most tube designers agree on is bypassing the cathode resistor. The tube can behave much better. I don't like the 6DJ8 and its variants; while they are quite linear, they are also very prone to microphonics. The schematic (if accurate) indicates it could really do with a grid resistor on the input stage. Since an EL84 is used, an ultralinear output transformer could also be used, resulting in lower distortion in the output section. I know a number of designers that don't like UL operation but as far as I can make out they are intentionally trying to make more distortion.

A PP 300b amplifier can make about 35 Watts which will be far more satisfying your speakers. 

@atmasphere thank you for your guidance. As an amp manufacturer yourself, you have been very helpful with the education. Quiet contrary to many manufacturers who come here mostly to advertise their products.

We all have different views as manufacturers and that is as it should be.  I don't give detailed advice because some of my views will clash with others and that is not what I really want to do on a public forum.  I will say from years of experience, that in general, the larger the plate on the tube, the better it sounds to ME, but that is not everyone's view.  I will also say, that if you know what you are doing, directly heated tubes are the way to go, but there are problems to solve and therefore they are not for everyone.  I will also say that I prefer amps with zero feedback, but @atmasphere and I will diverge there, and that is just fine.  I respect his opinion for what he builds.  I do agree with him that the 6DJ8 family isn't all that good sounding a tube.  Neither is the 12au7 and I never understood why people used them.  But we all have our taste.  I would still take a well designed push pull amp over a single ended one every day of the week, but again, that is my opinion and take it or leave it.  I also favor completely regulated supplies, but others differ in opinion.  

I am sure if you buy an amplifier you can modify it to improve it, unless you are buying a very expensive and well designed amp that someone has put a lot of time and effort into.  One that was designed without the usual budgetary constraints.  As a modifier of more cost effective gear you can certainly remove some of those constraints.   If there are coupling caps then those are generally modified for personal taste.  There are certainly things  you can do to power supplies on cheaper gear to improve them, but it depends on whether there is room.  Often just bypassing a power supply electrolytic cap with a small film cap of sufficient voltage rating can make a subtle improvement for a few dollars.  We used to do all sorts of things to improve vintage gear when rebuilding it.

I will also say that I prefer amps with zero feedback, but @atmasphere and I will diverge there, and that is just fine. 

@donsachs I wasn't a fan of feedback for a long time. Here are two things for you to consider: the first being that in most amps with feedback, tube or solid state, the feedback node (which might be the cathode of the input tube) distorts the feedback prior to its being able to do its job. In the case of a tube, the tube isn't linear. So using the cathode as an input for the feedback means the feedback signal gets distorted by the tube as it mixes with the incoming audio. So IMD, harmonics and inharmonic noise is generated, literally causing the noise floor of the amp to be composed of that rather than actual noise.

Norman Crowhurst wrote about this problem in the 1950s but didn't suggest a solution.

However the solution is simple: wrap the feedback around the amp in such a way that it can be mixed with the incoming signal using a resistive divider network at the input of the amp (in the case of a tube amp, at the grid of the input tube rather than using the cathode) so that the feedback signal isn't distorted in the mixing process. IOW, much the same way you see opamps do feedback.

One interesting tube preamplifier design that used this idea was made by Leak, called the Point One. Properly refurbished and given a good power supply its quite musical.

Of course, to do this effectively you usually have to have more gain that can be used for the feedback. That is why Leak used pentodes but I've seen Fisher circuits that used this idea too using 12AX7s.

The second problem is if the amp lacks gain bandwidth product, if you try to use too much feedback, at some frequency in the audio band distortion will rise since the feedback is being decreased on a slope of 20dB/decade. Bruno Putzeys wrote a fine paper about this problem. I'm convinced this causes brightness and harshness. This is a very common problem in amplifier design, although not nearly so now as it was up to about 20 years ago.