I had an epiphany today. Many of the technical problems we talk about in audio amps, including tube amps, are problems that only exist due to cost to manufacturers. The pressure to make an ever-increasing profit...for corporations (we really need to do something about corporations - they have all the rights of individuals, and virtually none of the accountability or liability each of us have)...or the intense pressure to just stay alive, for the privately held amp-maker garage-setup company...creates devil's choices of how to get good sound without spending so much on components and fabrication that profit goes right out the window.
These problems do not need to be an issue for DIY'ers. Yeah, we have cost issues, all right, and a favorite topic on tube forums is the "junk box" amp...it's FUN to take a bunch of junk that's piled up over the years and make a half-decent amp out of it.
But we don't pretend...I hope not...that the junk-box amp is GREAT. It's just reasonably good, and cheap, and a great way to make old stuff useful. One can always use another tube amp. Laundry room, garage, outhouse....
As DIY'ers, we have the wonderful opportunities to burn-up (or cut in half) "straw-men" all over the place. In this article, as I accumulate them, I'll be listing them here. Submissions are welcome; I like to attribute other authors, so you're welcome to submit, I won't steal your work, I'll publically thank you.
The first straw-man, which I've made a BIG DEAL about in other parts of this blog, is the matter of "blocking". (See the article "What Transformation Audio is All About", considerably far down the page; I don't want to copy it all here, because it would just slow you down here if you've already read it...later I'll put a link here about "blocking" if you don't know what it is). And I talk about direct-driving the output stage, with direct coupling, using a cathode-follower stage or mosfet-follower stage or TubeLab's PowerDriver circuit (see links on right)...but looking at Max's superb EL-34 power amp at AngelFire (see link near top on right)...I had this epiphany.
"Wait a sec....." I sez to myself, I sez, "Max drives the output tubes with cathode-followers (good)...but the cathode followers are capacitor coupled to the output EL-34's....why not direct-couple?....wouldn't blocking be a potential problem?"...and then it hit me: "Ahah!!!!!!!!! Blocking is only a problem in amps where you're trying to save cost by beating the crap out of a single output tube, instead of paralleling enough of 'em to avoid the problem altogether!!"
"Blocking" is only a problem if you're pushing the output stage tubes to the limit. If you parallel enough tubes (or enough mosfets, heh heh heh), you have so much reserve power - assuming you're running the amp at sane volume levels, of course - that the output stage never gets whacked to the ceiling or slammed to the floor. This means you can capacitor couple to the output stage without worrying about "blocking".
A beautiful example of Cutting the Gordion Knot! (see "Wonderful Links for LEARNING about tubes, electronics, mosfets & transistors").
Max wanted extremely low distortion in the output stage BEFORE negative feedback, so he wouldn't need to use much negative feedback (with his design, you could do without negative feedback altogether, if you want). How to do this? The same way you get low distortion with ANY tube: run the tube with only a small portion of it's potential signal-swing capability used. I'll put a diagram in here later to demonstrate what this means, you'll understand instantly when you see it. (I have to figure out how to do that in code; the blog-ware won't let me do it directly, but no big deal, just time).
So, Max used an ELEGANT solution (not a simple...as in "simple-minded"...solution...different than elegant): he used multiple output tubes in parallel, so he could get good power WITHOUT pushing the tubes near their limits. (He essentially made the output stage into more of a CURRENT device than a VOLTAGE device; by paralleling the tubes, he gets lower plate resistance, higher current, and a more efficient impedance match to the output transformer...I'll post an explanation of this, with math, later). Aaaaand how he doesn't need to push one tube to it's max....the quartet of EL-34's kinda loaf along, and transients, if they clip, are clipped softly, and don't cause "blocking". See? How cool, huh?
So, we now kinda have two legends/myths transmogrified (Calvin and Hobbes) into one: Cutting the Gordion Knot, and Straw Men: Cutting Straw-Men in Half with a Mighty Bronze Sword!! Cooooooooool.
More Straw-Men to come heh heh heh heh
Best, Charlie
