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In Reply to: RE: Most popular SET amp posted by mark.korda@myfairpoint.net on May 08, 2015 at 12:11:44
As far as circuits go, the Western Electric 91A clones, 300B and/or 2A3 seem to be quite popular. That may be because I have one and so I pay more attention to them.
I don't know of any manufacturer who makes and sells them, I believe every one of them is DIY.
Edits: 05/09/15Follow Ups:
I think the Joe Roberts build article on the 91A clone in SP #1 had a lot to do with that. For anyone that wanted to build a good SET amp (then or now) there it was with all the info you needed. Plus the 300B is a terrific tube now that there are other brands besides the WE available that sound good.
Edits: 05/10/15
Here is the article
DanL
How many of you built your 91 clones with that feedback loop included ?
That feedback was a terrific bandwidth extender. The first SE amps I made in the mid to late '80s were built on Thordarson T-30W08 chassis. Without that feedback loop, (also considered a tone control by some), the amps had flabby bass with little oomph. Add, or switch in that loop, presto change-O, better bass and dynamics instantly; especially with mid-grade opts like the early WE, Thordarson, UTC, Samson, General Radio, etc....even the early Peerless offerings to replace the WE 91 SE tranny...Keep in mind that theoretically, if a feedback loop is "before" the opt tranny, the opt is not considered to be part of the loop. Yet, sonically, the WE 91A (300A tube's plate to 310A screen grid) feedback makes quite an impact...
Also, looking closely at the Loftin-White scheme circa 1927-1929, the "raised" voltage on the opt tube filament(cathode) is connected to the front end input tube's plate...certainly considered a feedback loop by many...Connecting certain amplifier power supply caps' negative return lead to an output tube's cathode is also considered feedback by many soundmen...
So folks, please do not underestimate the potential benefits of all topologies...feedback included....
Do better OPTs these days take care of most of that?
Observe, before you think. Think before you open your yap. Act on the basis of experience.
Maybe a little...It is my understanding that the inherently high output impedance of the pentode driver causes the freq roll-off when driving a DHT. This is not necessarily a "bad" thing, and may actually be beneficial in some systems.
I run a tube DAC (real tube output, not buffer) and use a 6C6 to drive a 2A3 and there should be some roll-off - maybe -3dB at 20kHz - but I don't find it obvious; the high frequencies are resolved and plenty present for me. I could easily run some +ve feedback, but don't feel the need.
Regards,
91.
Edits: 05/11/15
I've seen what I believe is the "real" WECO 91A schematic, and there's a lot in it that is not in my clone. It's actually quite complicated compared to the one I built IIRC.
But I'm guessing that the feedback loop is the most impactful piece that's been omitted by most builders.
That amplifier kicked off the modern SET revival and its basic topology has remained popular - easily the longest-lived design. I agree that DIY is more central than built amps or kits, so it's the most comparable to the ST-70 kit. Still, the comparison is stretched given the enormous number of ST-70s sold back in the day!
Commercially available SET hifi amps are numerically small compared to all the other stuff out there. Just go to a hi-fi show, you'll see. I'd wager that the majority of the SET amps in the world are DIY; either kits, scratch-built, or very small scale manufacturing by artisan builders.
It's probably more useful to consider design topologies, rather than specific manufacturers, when trying to determine the most popular SET amps. When you look at it that way, the Western Electric 91A clones (or, more generally, pentode driver, triode output) are definitely among the most popular. Maybe even the most popular. Hard to say.
Another big contender for most popular would be the Loftin White type (or, again more generally, two-stage direct coupled). There are lots of these around the world. Maybe more than the above.
The only other one that comes to mind as super popular is the two stage, low mu triode input/driver coupled to triode output design. Probably not as many of these as there are 91's and LW's, but there's still a lot of them.
There are other designs (IT coupling, etc) that will find recommendation from many, but in terms of sheer popularity, there are probably fewer of these types of amps out there.
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