300b lovers

There was a funny incident a few years back at the Dallas Audio Show. Back then, it was a little bitty thing, just a few exhibitors, but very much a home-town thing where everyone knew other. New to me, of course, as a former West Coast guy fairly new to Colorado. Never been to Texas before.

I wander aimlessly down the halls, no real goal in mind, looking for interesting tube gear. I walked into one room, and whoa, that’s Nelson Pass! Now people joke about me being Mr. Natural, but Nelson really looks the part. You can’t miss him. Me, my only trademark at a show are the Hawaiian shirts I like to wear.

Nelson had actually built an open-baffle speaker around a Lowther and a 12" guitar speaker called the Tone Tubby that I had written about some time ago. Well, that’s different, but why not? As I turned towards the door, Nelson blocked the exit.  how do I get in these situations? Me and my big mouth.

It turned out the two drivers were bi-amped with a simple low-level crossover. Oh, now I get it. Four knobs, two for level, two for the crossover frequency. Nelson wanted me to tune the thing ... by ear.

Now I really want to escape, but Nelson is still in the way. Fine, anything to get out. Twiddle, twiddle. Too little bass. Mo’ bass, man. Turn that knob up. A bit less Lowther, but not too dull. Mess with the crossover overlap some, so that mellow hemp cone transitions into the characteristic hard paper Lowther cone. A few minutes later, sounds OK, as good as I can get it right now. (Did not sound OK when I walked in.)

Escape permitted. Afterward, Nelson allowed as to how he saw that article I wrote about the charms of the Tone Tubby and wanted to build a simple open baffle around it, with a Lowther on top. So he figured if anyone could tune it on the fly, it would be me. Well, he had me there, but I allowed that he might have different preferences than I did, so feel free to mess with the knobs, although he might want to mark the current positions before changing anything.

These weird things happen to me at shows. That’s how I met Nelson Pass.

Yes we all have different tastes.  I would say the Lynn and I overlap considerably.  When we discussed our favourite rooms at the PAF we were in general agreement.  I stand by what I said a million pages back in this thread.  If I were to magically create a straight wire with gain that could drive any speaker load with infinite power and had no sonic signature at all, my guess is that half the people wouldn't like it.   Heck, maybe I wouldn't like it.   If I were to characterize what I favour it would be lush, not mush, an ethereal high end where cymbals and triangles hang in the air and decay, and tons of detail without brightness.  I hate artificial, in-your-face detail created by bright sounding systems.   I am an imaging freak as well.  If I build an amp or preamp and the sound stage doesn't extend at least a few feet outside the speaker boundary and the vocal doesn't appear at the floor to ceiling interface, then something is wrong.   I was a little dismayed at the show when our system could give great depth, but we had a shoe box of a room and the sound stage was constricted in height and width by the room boundaries.  Same system in my room threw the sound stage I just described.  I voice things with acoustic instruments and vocals.  If you can get a piano right you can pretty much play anything else with correct tonality.   If you can follow individual voices in a choir or instruments in an ensemble of some type, then your system can handle complex passages without breaking down.

Oh, and I love a good horn system.  Not the cheap ones that are popular, built with junk parts and cabinets, but a good horn system.  

So that is what I like.  Lynn and I agree more than we disagree!

Since he is no longer with us to defend his design, what do you think are the positive attributes of a differential output stage in a tube power amp?

@jaytor Since Lynn isn’t going there I’ll take this one. The advantage is the differential effect reduces distortion in the output section and makes the output section easier to drive since it will have a bit more gain.

There are a lot of differential output sections in well known tube amps- such as the Dynaco ST-70. What makes it differential is the use of a common cathode resistor. A Constant Current Source (CCS) can help performance but isn’t needed for the gain stage (whether an output section or not) to be considered differential.

Despite Lynn’s remonstrations, if designed properly a CCS in the output section of an amplifier will not limit current right up to the full power of the amplifier; in fact if the output section isn’t differential, using a pair of cathode resistors rather than a common one, the output power is unchanged or even reduced. I’ve seen applications where the use of the CCS actually increased the output power by a few Watts since the distortion was held in check to a higher power level.

What might not be obvious WRT an output section is you can set up the cathode circuit regulator to sense B+ variation and adjust the cathode voltage in response, which reduces distortion and increases tube life. This eliminates the benefit of a regulated B+ which would otherwise be a hefty lift in terms of execution and cost. IMO Lynn is missing a bet on this one and leaving performance on the table.

@lynn_olson @donsachs @atmasphere - Thanks for your responses. My system is bi-amped with a SS amp driving the lower three octaves, so I'm primarily concerned about upper bass on up. I'm pretty happy with my current DIY 300B PSET monoblocks, but I like to tinker.

I've primarily been building electronics based on other's designs (at least at the module level), but I've recently been trying my own designs (heavily influenced by others). I think I want to build a PP amp as my next amp, using either 300Bs or 2A3s. My main speakers are pretty sensitive (98db/w line arrays) so I don't need a ton of power. I'm still thinking about what approach I want to take. 

I have a question about the first 6sn7 stage in my 300B SET. What is the best way to make load and coupling?
1. The drawback of the interstage transformers is a narrow bandwidth in combination with 6sn7 that has high internal impedance. But low distortions.
2. RC - coupling has wide bandwidth but higher distortions.
3. LC - coupling looks good, but what is the optimal inductance for 6sn7? The issue can be not deep enough low frequency bandwidth and high LC resonance Q.
4. What are the drawbacks of SRPP with C or direct coupling? What is for and against it?