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

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.

Or you can simply use speakers that will run happily on 10 watts and build a very simple (well not that simple actually if you consider power supplies and transformers, etc..)  push pull amp with no feedback that is flat from 20-20K with very low distortion to drive them and be happy:)  Using  DHTs...  to each their own.  The problem lies in supporting people that need more than 20-25 watts/ch.  If they have made that speaker choice so be it.   We all have our paths to audio nirvana....

There are oodles of 20 to 35 watt push-pull amps from the classical era of the 1950’s through the 1960’s. Most of the "mods" of these amps are nothing more than updating the coupling caps, which is a good idea anyway with an old electronic product.

A much bigger challenge is re-designing the power supply, which usually means a new, roomier chassis without the period charm of the original. The output transformers, though, are often very good and worth retaining. But this does mean the destruction of a desirable vintage amp, which some view as cultural destruction, like ruining a 1956 Chevrolet in good running order. I personally do not approve of destroying vintage equipment in good working order.

But there are plenty of modern amps which take the same approach, using vintage circuits, and with good output transformers (which are NOT cheap, often costing more than an entire Chinese-made amp). I can heartily recommend the Kootenai (KT88) and Valhalla (6L6) amps made by Spatial Audio Labs under license from Don Sachs. They are best classical-circuit amps I have ever heard, superior to any restored vintage amp, and priced very competitively. If you are extremely price sensitive, you can’t go wrong with Quicksilver, which are serious amps, made in the USA, and by a guy who knows what he is doing.

P.S. Building a good PP 300B amplifier is surprisingly expensive. Sure, you can adapt a 1950’s circuit, but a 300B is far harder to drive than an EL34, 6L6, or KT88. The modified 1950’s circuit will have high distortion in the driver portion, which then dominates the sound of the amplifier. This is especially true if 12AT7, 12AU7 or 6DJ8 nine-pin miniature driver tubes are used. Put another way, vintage circuits from the 1950’s or 1960’s, which are designed for beam tetrodes or pentodes, are not well suited for 300B triodes.

I read something years ago that stuck with my and I have always tried to heed.  A really good tube amp is the sum of three things.  How good is the circuit, how good is the power supply, and how good is the iron (transformers).  Those who favour OTL designs can ignore the iron.  Of course there are other factors at play such as how much distortion there is in the tubes chosen, but if you use decent tubes and pay serious attention to the main three points, you should end up with a very nice amplifier.

I rebuilt hundreds upon hundreds of vintage tube amps and preamps before I started making my own.  You could tell the good ones before you even started.  If there were serious compromises in power supplies or transformers, then no amount of replacing old parts with better modern ones would ever make them great amps.  You could tell the great amps from the schematic and quality of parts used.

So my advice is the build the best power supply you can, use the best iron you can, and find a good circuit.  I exclusively build push pull amps.  I don't build single ended or parallel single ended circuits.  I understand there are those who favour such designs.  I only use regulated supplies, and I also only use tubes with larger plates, and as many directly heated tubes as I can.  Those are my choices based on the main rules above, and moving beyond the "conventional" designs I built for years.  The conventional amps I built are very nice, but the latest full balanced circuits are far superior to my ear, and more costly to build.

So my advice on a good 15-20 watt 300b or other amp, is to heed the three main rules, and build a good push pull one.  If you can DIY you will save lots of money.  There are many great parts from companies like Neurochrome, the Rod Coleman regulators favoured by a pretty large camp, read Barola valves site for some great info...particularly on drivers.  There are lots of places you can check out for a DIY 300b project.