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

How do I know how much power my SET amp is using?

That's the tricky bit. The easiest way these days might be to use a sound pressure level app on your phone, once you've calculated how loud 0.75 Watts would be driving your speakers in your room.

The AER drivers are measured at 1 meter. Sound pressure level drops off the the square of the distance, but in an enclosed room not so much due to reflections. The size of the room, the distance the listening position is from the speakers and how lively the room is all make a difference. If an average room you're likely alright if the sound pressure is below 90dB or thereabouts.

 If you use a Hammond, you will never hear what the circuit can do....

@donsachs Great for small guitar amps though!

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....

@donsachs I find a mark of the best systems is they don't sound loud even when the are- relaxed at all volumes. On that account, I've found that even with very easy to drive speakers (98dB, 16 Ohms) you tend to use a lot more power than one might think!

But at the same time I've also found that the better the amp is at making clean power, the better its first Watt is, that such an amp can be quite convincing.

These two things might seem opposites of each other but they are not.

I designed a little 5 Watt PP amp about 4 years ago that was meant for my bedroom system, because I couldn't find a compact amp (this one can sit on a sheet of notebook paper with room left over) of the same power that wasn't junk. 5 Watts doesn't seem like much (although for a desktop, bedroom system, headphones or a system with really efficient speakers its plenty), but compared to a 5 Watt SET its quite a lot since it has a lot more usable power and is overall more musical than any SET (for example it doesn't sound 'loud' which many SETs do even though they don't make much power) I've heard. I was finally talked into producing that amp about a year ago.

It uses feedback. But one of its tricks is that it didn't need feedback to sound just fine. I mostly used it because it had too much gain (with only 3 tubes/channel).  

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.

@lynn_olson I refurbished an Eico EL84-based amp recently. That one didn't have good iron so output power was rolling off at 30Hz and distortion at bass frequencies was high. You do  have to be careful about what vintage amp you try to 'renovate', 'update', whatever... many of them are built to a price point since back then, they were manufactured as a commodity so output transformer bass response is a likely candidate for cut corners.

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.

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.

Ref atmasphere's post. It depends on how much distortion one is willing to tolerate. An SE amp with a pair of 300Bs will not put out 15 watts @1%thd.

Yes- if 15 Watts the distortion will be more like 10%.

Have gone through some amps, like FW SIT3, Pass X25, LTA UL and some other Class D but in all cases it seemed to lack dynamics

@debjit_g This is a very common comment about single-ended power amps! Single-ended amps (SET or PSET) seem to have more 'dynamics' because distortion on the leading edges of the transients is interacting with your ear in a particular way- the ear interprets the distortion as loudness because it uses higher ordered harmonics to sense how loud sounds are. Those amps with less 'dynamics' are that way because they are lower distortion.

This is why I mentioned that SETs only have about 20-25% usable power. Above that level (generally speaking) the distortion masquerades as 'dynamics'. So if you want to hear them at their best, the speaker needs to be more efficient. You can get around this problem if the SET (or PSET) has feedback, but then you find out that the amp is under powered! This is why I've been recommending something push-pull.  A PP 300b amplifier can make about 35 Watts which will be far more satisfying your speakers. 

my speakers Cube Audio Nenuphar is a single wide band driver. The manufacturer recommends amps with 0 or low feedback design, which will also have low damping factor.

@debjit_g Here are some more things to think about.

I’ve worked a lot with such drivers. Here’s a tip: supplement the driver with a subwoofer. If you can, also do something to prevent bass from getting to the driver! You’ll find the mids and highs to be a lot more coherent, especially at higher volume levels (this is due to the very audible effects of Doppler Effect distortion).

If the amp you use is an SET, it will benefit quite a lot from not having to reproduce bass as well. SETs really have troubles making bass anyway due to the requirements of the output transformer; it gets technical but in a nutshell this problem stems from the need to prevent distortion caused by the output transformer itself (BTW a PP amp does not have this problem). As a result no SET can make full rated power at 20Hz; if they did the output transformer would be the size of a small refrigerator!

This bit could be easily taken care of by the use of a smaller value coupling capacitor in the SET prior to the output section, with the intention of rolling it off at 60Hz or so. We’ve done things like that for our customers before (although we make an entirely different kind of amp known as an OTL)- so it might be worth it to talk to whomever makes the amp you settle on about that.

Since your thread title doesn’t seem like you are limiting yourself to an SET, IMO/IME your amplifier investment dollar will be a lot better served by a PP amp and there are PP 300b amps like the excellent Spatial Audio amp designed by Don Sachs.

I’ve been investigating SETs since the early 90s; they have quite a few problems which really limit how they can be best used. If you are in a normal room for example, unless your Cube driver is horn loaded, it does not have the efficiency to really show off what an SET can do with the power you ask about in the thread title. Typically high efficiency extended range drivers like the Cube might be 99dB at best (unless horn loaded) and since SETs only have 20-25% usable power, you really only have about 4 Watts to play with.

To give that some perspective, my speakers at home are 98dB and 16 Ohms (nearly the same as yours), so very easy to drive, and I’ve found that they need a good 50-100 Watts in an average size room. In this way the amp doesn’t have to work hard at all and due to the extra cleanliness of the sound, its very natural to use more amplifier power. If I put in a lower powered amp like 15 Watts, the fact that it is more strained is immediately apparent compared to the more power amp.

For everyone here who is considering a zero feedback 300b amplifier, something you should know which has to do with the loudspeakers you plan to use: http://www.atma-sphere.com/en/resources-paradigms-in-amplifier-design.html

The short explanation is this: tube amplifiers with no feedback don’t adhere to the Voltage drive principles of about 99% of all loudspeakers made. That 1% left over are nearly all made by high end audio loudspeaker manufacturers. Zero feedback tube amps try to make constant power with respect to the speaker impedance rather than constant Voltage. This means if you mix the two technologies you’ll get some sort of tonal aberration.

In particular if the amp in question is also an SET, plan on having a speaker that is a bit more efficient! This is because all SETs that are zero feedback only have about 20-25% usable power- above that power level distortion causes the amp to sound ’dynamic’.

You might think that ’7 Watts is enough’; here’s a tip: Its the mark of the best systems that they do not sound loud even when they are. If a system sounds loud when a sound level pressure meter (available as an app for any phone) shows that it really isn’t, what is happening is distortion is being interpreted by the ear as sound pressure.

Put another way, if your speakers are efficient enough that the SET never needs to make more than about 25% of full power you’ll really start to hear the best of what that amp can do. In case there’s any doubt, this is part of why horns were so common in the 1950s and prior.