Home » Audio » Thermionic Emissions » Ping Steve Brown -- re: Seth(s)
Ping Steve Brown -- re: Seth(s) [message #10017] Wed, 28 November 2007 19:02 Go to next message
Skip Pack is currently offline  Skip Pack
Messages: 51
Registered: May 2009
Location: Hollister, California
Baron
Steve,

I noticed in other noisier places that you seem to be getting ready
to put a Seth together. I also noticed that you had done a 6B4G
version a couple of years ago. Since I'm collecting bits for one,
prompted by a Magnequest clearance of S240s a couple of weeks ago, I
thought I'd check in with you. I currently expect to build the signal
circuitry to the Mark II spec, but possibly do a different power
supply. I have several 500 VA step up/down power transformers that
would give 234 and 468 secondaries (AC), and thought I could use two
bridges to make the 320 vdc B+ for the 2A3s and around 420 for the
driver stage. It this excess complication without commensurate
benefit?

For those unfamiliar with this amp check this link:

http://www.magnequest.com/diy_lessard_2a3pp.htm

I have a DAC to assemble before I start on this, just collecting
pieces now.

Thanks,

Skip

Re: Ping Steve Brown -- re: Seth(s) [message #10034 is a reply to message #10017] Sun, 16 December 2007 07:12 Go to previous messageGo to next message
SteveBrown is currently offline  SteveBrown
Messages: 330
Registered: May 2009
Grand Master
Skip, sorry I didn't see your note sooner! Yes, I've built a "SETH" style amp, got one sitting on my floor right now (though fitted with 6L6's not 2A3s). If you're going to use 2A3's, you only need to swing +/- 70v or so at the driver, so no need to give it a 460v supply. I don't know what driver tube you will use, but if you stay with something that has high transconductance like the design calls for, at say 150v on the plate, you don't need more than a 300v supply for sure - in fact, that would even run a 300b pretty well (Gordon from Wavelength Audio has advocated this for using a WE417a for 300Bs and I've built it that way, works great). As for the 6B4G's, I didn't have good luck with hum on them. This is related to the higher heater voltage and not using DC on the fils. I'd recommend staying at 5v or less (that is, use a 2A3 or 300B). The Sovtek single plate 2A3 is a very nice tube, and not all that expensive. By the way, I had MQ do my EXo-173's w/some nickle lams and I really like the way it sounds. Not sure what the standard sounds like. Best of luck!

Re: Ping Steve Brown -- re: Seth(s) [message #10037 is a reply to message #10034] Mon, 17 December 2007 07:46 Go to previous messageGo to next message
Skip Pack is currently offline  Skip Pack
Messages: 51
Registered: May 2009
Location: Hollister, California
Baron
Steve,

Thanks for the reply. Too many sites these days, it's easy to miss
something, somewhere. In any case, I was wanting to isolate the driver
stage from the output stage, and since I have 2 220 VAC secondaries on
the power transformers, I was originally thinking to put one bridge
across one of the secondaries, and then another across both in series.
I have since seen a discussion on AA that suggests that this would be
catastrophic (though I don't quite see how). If I had the 400+ B+ for
the driver, I would drop it with a large, hot resistor and maybe a
choke to further isolate that stage. I suspect that simply using one
secondary with a CLCLC for the output at full voltage and a choke
loaded LCLC(maybe one more LC) for the driver would work as well or
better and avoid the higher voltage complications.

Skip

Re: Ping Steve Brown -- re: Seth(s) [message #10038 is a reply to message #10037] Mon, 17 December 2007 15:32 Go to previous messageGo to next message
SteveBrown is currently offline  SteveBrown
Messages: 330
Registered: May 2009
Grand Master
Hmm.. not sure why you can't do what you proposed, either. Guess you could experiment with a couple filament transformers to see what happens... You could also consider stacked supplies, but that does not totally isolate either of them. I had a transformer custom wound one time that did have seperate windings for each stage, plus bias supply. Turned out I way over spec'ed it and it was mamouth, and cost me an unexpected fortune. BUT, it was very nice to have the seperate supplies if for no other reason than you could work on one part of the amp without impacting the DC loading you'd already figured out for the other part. Since then, and FWIW, I've found I prefer to have things tied to the same supply for sonic reasons. For some reason it sounds better to me. Call me nuts. That includes L & R channels, too. Right now, for example, I'm running a PP EL84 amp that uses a differential driver. Both L & R channels have the same supply. Does it compress the soundstage? No, it's the widest soundstage I've ever heard on an amp - almost spooky in some respects. By the way, the soundstage on this amp is MUCH better (as is the bass) compared to my SETH. The SETH gives a narrower soundstage. I suspect if I played with the parafeed cap I could introduce some bass boost, but I've not done that yet.

Parafeed Cap? [message #10039 is a reply to message #10038] Mon, 17 December 2007 19:00 Go to previous message
Skip Pack is currently offline  Skip Pack
Messages: 51
Registered: May 2009
Location: Hollister, California
Baron
I assume you mean the coupling cap before the autoformer between stages,
or are you playing with parafeed push pull?



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