Tubelab Simple SE

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Tubelab Simple SE

Postby lousymusician » Sat Jul 28, 2007 11:50 pm

Tubelab been mentioned in a few other threads, but I figured it was time to start a separate thread.

For background, Tubelab.com is run by an engineer in Florida, who is selling circuit boards for two single-ended amplifier designs. The Tubelab SE is designed for directly-heated triodes (45, 2A3, 300B), but breaks with tradition by using a MOSFET to drive the output tube grid. The Simple SE is designed for indirectly-heated output tubes (EL34, KTxx, 6L6), and uses a SS constant current source for the driver stage.

I ordered a Simple SE board some time back, and got it running this evening. mine uses triode connected El34's (Svetlanas, for now), an EH 12AT7 driver, Transcendar OPT's (from ebay, but wound just down the valley in Hollister), Sonicaps for coupling, a Triad choke (which fits under the chassis) and the inexpensive Allied power tx that the designer uses in his own amps. The Panasonic electrolytic filter caps are bypassed with a 100 uf motor run, also from ebay.

For a chassis, I bought a heavy aluminum roasting pan from Linens & Things. Since the pan is used bottom-up, I removed the handles and remounted them upside-down. After drilling all of the mounting holes I painted it with Rustoleum gray hammertone. It's a nice sturdy chassis, and wound up looking pretty decent IMHO. I'll get some pics up one of these days.

As for the sound, it's currently hooked up to my Hemp Acoustics / Metronomes burning in. Compared to my modified ST70 (which uses the same output tubes in push-pull triode), the bass is a little softer and 'rounder', but the SE throws a better soundstage. The speakers disappear more convincingly. It'll be interesting to see how it develos as the amp burns in, but for now it's sounding darned good!

More later,

Bill
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Re: Tubelab Simple SE

Postby Eddie Vaughn » Sun Jul 29, 2007 1:56 am

Hi Bill,

Sounds like you've got the fixins for a right nice amplifier there! I've read several positive impressions of the Tubelab kits, and those Transcendars you're using are certainly nothing to sneeze at. That's important considering that (IMHO) 50% of a SET amp's sound is in the output transformer quality. Transcendars don't produce as much bass as some can, but once they're burned in well it's tighter, cleaner bass than some brands that cost much more. They have great microdetail, and their midrange tone is simply superb by anyone's standards, one of the best IMO.

lousymusician wrote:Compared to my modified ST70 (which uses the same output tubes in push-pull triode), the bass is a little softer and 'rounder'......


You can expect that, as the ST-70 is using 20dB of NFB (unless you've changed it) and trioded power tubes, especially if you've fixed the design flaws in the feedback loop and put in bigger coupling caps. Its output Z is probably only a couple tenths of an ohm, versus around 2 ohms for the zero feedback EL34 SET (depending on the EL34s' operating point and the output transformer impedance ratio). Not considering output transformer secondary winding resistance or other variables, that's a DF of 40 versus a DF of 4.

Having said all that, the Tubelab's bass will undoubtedly tighten up a lot and gain significant extension with time. If the ST-70's feedback loop and coupling cap values haven't been modified from stock, I'd be willing to bet the SE amp will actually have cleaner bass once fully burned in, despite the very low DF (depending on what speakers you're using, of course).

Congratulations on a successful build!

Eddie :D
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Postby lousymusician » Sun Jul 29, 2007 4:22 pm

Thanks, Eddie!

All that's left of the ST70 is the chassis and iron. Otherwise, it has Joe Curcio's input circuit (2 6DJ8's per channel, each cascoded, in a CCS differential amp), and regulated PS. When the quad cap went south, I replaced it with four big Solen film caps, so there are no 'lytics in the B+ supply. It has always sounded pretty good to me.

I expect (or at least hope for) the SE to have a different set of virtues. I can live with a little less bass definition to get a better midrange. A side benefit should be less heat, a real factor in a small room in the summer! So far, I'm pretty pleased.

Bill
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Postby kiwi » Sun Jul 29, 2007 5:12 pm

Hi Bill,

How much are the boards?

Cheers

Chris
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Postby Robert » Tue Jul 31, 2007 6:57 pm

The ST70 boards or the Tubelab boards?

The Tubelab boards are $35.

I don't know what Joe Curcio's boards cost.

Robert :)
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Postby nodiak » Thu Oct 11, 2007 5:33 pm

Bill, if you have pics of the build I'd love to see them. Also any comments on sound, tube rolling, etc. I hope to get to my SimpleSE in the next few months and could use the inspiration, my first amp.
I've been out of the audio world for some months and see the Transcendars are no longer on ebay, I'll see if I can find a pair.
Don
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Postby Nuuk » Thu Oct 11, 2007 5:49 pm

For a chassis, I bought a heavy aluminum roasting pan from Linens & Things.


http://myweb.tiscali.co.uk/nuukspot/decdun/gcgallery4.html

Take a look at that last GC on that page! :wink:
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