Recommendations on stuff to look at for upgrading a 300B SET amp

@jbhiller

What you are up against is that to make more power, SETs have to sacrifice bandwidth. 300b amps are at about the limit of what can be considered 'hifi'; even then its very difficult to get them to go from 20Hz to 20KHz and most of them don't.

Because a horn system is about the only thing that has enough efficiency (or in some cases, single driver full range speakers like Lowther) to work with a 300b, this isn't too much of a problem because most horns and full range drivers can't make much bass below 40-60 Hz anyway.

Now of course you will hear of a lot of people running speakers that are less than the sort of efficiency you need (+99dB) but they really aren't getting the best performance out of their amps as a result unless they are listening in a smaller room. To really take advantage of what SETs bring to the table, you really don't want to push them past about 20% or so of full power, otherwise they start sounding 'dynamic' but this is entirely caused by distortion. BTW, once you know this fact, its easier to hear that such is the case so I may have ruined it for you- if so, my apologies.


The ear uses higher ordered harmonics to sense sound pressure, and SETs generate more higher ordered harmonic distortions once they start getting over about 20-25% of full power. Since music has lots of transients, as you turn up the power these harmonics initially are showing up on the transients of the music- and that is how they sound more 'dynamic' than one would expect for a lower powered amp.


If you really need more power (if a more efficient speaker is out of the question), IMO/IME you are far better off looking for a lower powered push-pull tube amplifier. You'll hear more detail and bandwidth won't be a problem. Most such amps won't be as lush since they don't make as much 2nd order harmonic distortion, but overall they stand a chance of being more musical than a larger SET.

So, how about changing your speakers to something more efficient? Or, looking at other tubes like GM70 or 211?Check out Lampizator and NAT.

@atmasphere , I know you to be super knowledgeable and I really appreciate what you are saying--so much so that it’s making me rethink the tube world, even if just for today. My speakers are 99dB efficient by specs. Maybe I’m just better off staying put.

I loved the push pull Primaluna with my PSB T2s (89dB) but I ventured into the 300B world and loved its intoxicating recreation of vocals. Maybe the solution is just to run two systems?

I almost know you have to be right about the 20-25% rule just by what I hear, which is nothing compared to your experience level as a builder of fine amps.

Yes- I get what the allure of the SET is all about. That inner detail they are known for comes out of the fact that as power levels are reduced, the distortion decreases to unmeasurable. When the distortion is gone, detail is revealed. There aren't many push-pull amps for which that is the case, but there are a few.

The problem is that you can't get much bigger than the 300b; people have built SETs with dual 300bs that aren't too bad but they are only 3dB more powerful (which is to say, twice the power); the problem is that the way the ear hears, 3dB just isn't that much louder. If you really want to play things louder you need at minimum 6dB more power, which is 4x more power than you have now.  A 30 watt push pull amp that sounds nice isn't that hard to find, but an SET with that sort of power is challenging.


Its that, get more efficient speakers, or live with what you have....