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Schiit Happened: The Story of the World's Most Improbable Start-Up
Apr 3, 2018 at 11:15 PM Post #31,081 of 149,153
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Apr 4, 2018 at 8:52 AM Post #31,083 of 149,153
Jason Stoddard said:
Update to the update: I am an idiot.

The reason the coaster wasn't working was that the regulators were falling out of regulation. I'm so used to working with LDO regulators these days, I was caught out by the 317s needing a full 3V to regulate. They were just bareeeellllyyy on the edge of falling out of regulation, hence the buzzing-that-comes-and-goes phenomenon.

I only found it after taking the board upstairs and plugging the wall-wart into a different variac (that was set lower than the lab downstairs). LOUD buzz. Turn up the variac: no buzz. The good news is this is an easy fix, the bad news is that it means we're looking at 30V/15V rails, which will reduce the maximum output of the amp a bit, and change some schematic values. On the other hand, if I want to go realllllly simple, I could go to fixed regulators, but fixed regs are pretty mediocre until you go exotic (that is, far beyond the LM/MC7815 series).

So, maybe you'll see a chapter after all!
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So, you have validated Murphy's Law. Sometimes the simplest of problems-----well, you know what I mean.
 
Apr 4, 2018 at 12:47 PM Post #31,085 of 149,153
My google-fu has failed me.

Do the LISST tubes for Freya have about the same, more, or less gain than the supplied Tung-Sol tubes when used in quad configuration?

Thanks
 
Apr 4, 2018 at 3:15 PM Post #31,087 of 149,153
Apr 4, 2018 at 4:12 PM Post #31,089 of 149,153
Hey all,

So this is interesting: lowering the plate voltage on the coaster amp definitely put it in a different category altogether in terms of THD--and not in a different, better category. At least if you want something fairly linear, that is. Long story short, if you want a really, really tubey-distortion amp, building this guy with the 6088 tubes I started with will probably float your boat.

How distortion-y? Well, with the lower rails and 6088 tubes, we're talking 1.5-2% THD at 1V RMS out. Yeah, that's huge. Sonically, it actually isn't that bad. I could see lots of people liking it. But we are getting well above the threshold of audibility here.

And remember, Vali 1 managed 0.3% or so, but with 2X the rail voltage. Now, I'm not gonna put 60V on a coaster. Nope. Sorry. That's a lot of volts for an open-frame amp.

So what to do?

Well, I could simply accept that I didn't reach my design goal of under 1% THD at 1V RMS. That's certainly a possibility. I mean, this thing is intended to be a coaster, really. And that's how 99% of them will be used, I expect.

Or, we could change tubes. How about the 6418, which is rated for 30V plate maximum?

I had some 6418 tubes, and sure enough, they're running happily in the same circuit at 0.4% THD at 1W. Not bad! Pretty close to Vali 1, in fact. This is probably how I'd build it, though of course you could also wire up some Russian rod pentodes or probably a half-dozen other alternatives. When I get a bit more time, I'll write up the final chapter, and give you the design values for both the 6088 and 6418 tubes. This will probably be next week.

Thanks for bearing with me on this.

All the best,
Jason
 
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Apr 4, 2018 at 4:28 PM Post #31,090 of 149,153
Jason Stoddard said:
Or, we could change tubes. How about the 6418, which is rated for 30V plate maximum?

I had some 6418 tubes, and sure enough, they're running happily in the same circuit at 0.4% THD at 1W. Not bad! Pretty close to Vali 1, in fact. This is probably how I'd build it, though of course you could also wire up some Russian rod pentodes or probably a half-dozen other alternatives. When I get a bit more time, I'll write up the final chapter, and give you the design values for both the 6088 and 6418 tubes. This will probably be next week.
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Any suitable tubes that sound okay and are still in production? Might help with the cost?
 
Apr 4, 2018 at 4:33 PM Post #31,091 of 149,153
Alcophone said:
Any suitable tubes that sound okay and are still in production? Might help with the cost?
Click to expand...

There are no submini tubes in current production--no demand for them. 6418s are $1-2 though, not a big deal.
 
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Apr 4, 2018 at 4:46 PM Post #31,093 of 149,153
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Apr 4, 2018 at 4:57 PM Post #31,094 of 149,153
@Jason Stoddard ... fascinating ! .... "And that's how 99% of them will be used, I expect." ? This is looking much better than that ... to me ! I can imagine demoing this to my sceptical (miserly) friends when this launches ! Great marketing !
 
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Apr 4, 2018 at 5:28 PM Post #31,095 of 149,153
Jason, a chapter someday on the A2 boards for Yggy if that was an in depth collaboration with Mike would be fascinating. About how, and how and why you decide it's time.

The upcoming donut adventure does mesh with this threads theme, but ouch, probably not compositionally inspiring for him!

Thanks
 
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