Pre Amp?

a DIY, modified Mullard 5-20 monoblock design

PREAMP UPDATE

Postby HOGHEAD » Thu Dec 21, 2006 2:21 pm

Hi Shannon, just would like to know if there is any preamp in the near future. Thanks, Jim
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Re: PREAMP UPDATE

Postby Shannon Parks » Fri Dec 22, 2006 5:50 am

HOGHEAD wrote:Hi Shannon, just would like to know if there is any preamp in the near future. Thanks, Jim


Funny you should ask this. Latest project on the burner is a 6B4 SET amp - and I actually intend to finsh this project! Yellow_Light_Colorz_PDT_16

The amp uses a driver board that contains the whole power supply, a 6AX5GT rectifier (think a 5Y3 with indirect 6.3V filament, 1.2A) and a 6SL7. The other morning, I realized that the driver can be used alone as a tube rectified linestage using a 6SN7. So it would be a two tube, tube rectified, choke filtered, all octal linestage. It could use any small power transformer with a 6.3V, 1.8A tap. The power supply can be adjusted for any secondary output. You could add as many inputs as you wanted and a selector switch. Any takers? Mine is already built.
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Postby TomMcNally » Fri Dec 22, 2006 10:55 am

I'm ready ! I have one NOS 6B4G ... time to look for some. Is anyone making new ones ?

... tom
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Postby Shannon Parks » Fri Dec 22, 2006 12:46 pm

Sovtek does and they are sweet monoplates (just like their 2A3s).
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Postby EWBrown » Tue Dec 26, 2006 7:58 am

I was going to mention the Sovteks, and there is also a (less than scrupulous) vendor who takes 6AV5GTs, and re-wires and re-bases and re-labels them into a pseudo-triode mode, so be wary of those. If I can find his name, I'll post it here. Caveat emptor, etc...


The 6AV5GT is basically a "dumpster tube" but it does have some interesting possibilities. Another useful plinker, though I haven't tried any of them out, yet.

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Postby mesherm » Tue Dec 26, 2006 10:22 am

I was running 6AV5GTs in my universal amp and they sounded decent, at least as good as the current batch of 6L6 types. Like most of the sweep tubes I have tried they need high bias voltage when you run B+ voltages above 300v.
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Postby EWBrown » Tue Dec 26, 2006 11:39 am

I'd SWAG that a 6AV6G in "triode mode" over with B+ 300V would probably have a bias around 50V, which isn''t too far from where a 2A3 (or 6EM7 power triode) runs.

I'll have to run it in Tubecad sometime (I finally got around to getting with the program)

The big "plinker pile" yielded several 6AV5Gs and their odd-filament-voltage brethren.

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Postby TerrySmith » Tue Dec 26, 2006 5:21 pm

So is this going to be on a PCB ? I'll take one!

Wonder if you could build a 1626 darling with it?
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Postby EWBrown » Wed Dec 27, 2006 6:32 am

You might have to play around with the B+ and filament voltages to accomodate for the 1626s (use a 12SL7) , but there isn't any real reason that this PCB couldn't drive 1626s, 2A3s, 6A3s, 6A5s, 6CK4s, 6AH4s, 6S4s, 12B4s, and even 300Bs, with the appropriate filament voltages.


/ed B in NH
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Postby Shannon Parks » Wed Dec 27, 2006 6:52 am

FWIW, the rectifier and the driver have separate filament connections, so you could use a different filament voltage for the rectifier, driver and power tubes. Cool, eh? Some small jumpers could let you use a 5Y3 or 5AR4, too. But for a small amp, these 6AX5GTs are very cool!
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Postby skidave » Wed Dec 27, 2006 5:31 pm

Yellow_Light_Colorz_PDT_12

I have a suggestion for a Preamp project. Last year I purchased Bruce Rozenblit's book 'Beginners Guide to Tube Audio Design' and I found it an excellent source of knowledge (theory and circuit explanation). In the back were a few projects and one was a preamp article he published in Glass Audio (Volume 3, Number 2 1991). This was my first dive into tubes and I thought of purchasing a Foreplay III. Personally, I thought it was a bit expensive, I did not like the wood enclosure, and I wanted stereo volume controls. So why purchase the unit to modify it further... I picked a nice aluminum Hammond enclosure and built Bruce's preamp. It is a simple two tube 12AU7 preamp (one per channel). One half of the tube is a a current source and the other is a gain stage. The circuit has a few nice tricks and after I shoved it into the enclosure that turned out to be a bit too small (should have picked the next size up) the unit is in my living room being used full time. I was expecting it to be very noisy because of the size of the enclosure and H.V. around, but I grounded everything carefully and no problems. I will take some photos and attach them this week.

I'm used to fixing and rebuilding high power transistor amplifiers, but this was a nice intro to tubes. I'm trying to talk the wife into letting me build some Mark III clones for my Magneplanars...

I don't know what type of permission we would need to post a schematic, but I'm sure several Group members have this G.A. issue.

Stay tuned for some photos. I'm hooked on tubes and just purchased three more books on tube audio design.

Thanks!

Dave
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Postby TerrySmith » Wed Dec 27, 2006 6:40 pm

This is going to be a very interesting project! I like the idea of having the outputs off the board, so you could try every different type of tube possible.

I might even try TV sweep tubes and compactrons just to see what happens. Yellow_Light_Colorz_PDT_06

What will be the ETA on these beauties?
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Postby Shannon Parks » Thu Dec 28, 2006 6:58 am

TerrySmith wrote:What will be the ETA on these beauties?


Probably will order a second prototype board next week with the DC filament mod. Then I'll test that for a week or two, then make a production order. So maybe 2/2/2007? Groundhog's Day!
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PREAMP

Postby HOGHEAD » Sat Dec 30, 2006 12:02 pm

IF YOU SUPPLY IT-I WILL BUY IT!!! Count me in Shannon and a Happy New Year to All!! Yellow_Light_Colorz_PDT_16
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Postby HOGHEAD » Thu Jun 21, 2007 4:59 pm

Any word on a pre amp?
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