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Old 7th October 2007, 11:46 PM   #1
240z4u is offline 240z4u  United States
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Default I want to build a preamp for my dad.. thoughts?

I would like to build a DIY preamplifier for my dad. He was using a carver preamp that blew up and needs a replacement.

I have a pair of 12b4s, and a set of 3 6sn7 tung-sols. I have 4 octal sockets and various other stuff from equipment I have stripped.

Can someone give me a suggestion as to which way to go?

I am no genious when it comes to building. I do understand circuts better then I did. I did a pair of Dynaco MKIII monoblocks recently. Something not overly complex would be good.

I am trying to keep a parts budget around 200 bucks if possible.

Thanks guys - Evan
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Old 8th October 2007, 12:07 AM   #2
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Evan,

If all dad uses as a source is a CDP, it's "a piece of cake". Those 12B4s WILL do the job. OTOH, if dad has (as I suspect) a collection of vinyl, you'll have to construct a phono preamp too.

I've attached a schematic for a 12B4 based line stage. You don't have to get quite that fancy with the parts to get excellent results.

BTW, the interlocked circle symbol stands for a constant current source (CCS).



Edit: added CCS remark.
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File Type: jpg 12b4_preamp_eliduttman5.jpg (64.6 KB, 1309 views)
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Old 8th October 2007, 12:30 AM   #3
gerryc is offline gerryc  United States
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If you have access to the March 2001 issue of AudioXpress magazine (or get a back issue), there is an article by Joseph Norwood Still entitled "High-Quality ($180) Control Unit". It should do all you want it to do. It uses different tube types than you have, but if cost is true to the title, it's within your $200 budget. It uses one 12AX7 (both sections paralleled), one 12AT7, and one 5687 (both sections paralleled) per channel. It includes an RIAA phono stage, switching for various line level inputs (CD player, tuner, etc) and a "Walkman" input. It doesn't have bass and treble controls though, if that's important.
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Old 8th October 2007, 01:09 AM   #4
240z4u is offline 240z4u  United States
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This is going to go in my dads bedroom system so phono stage is not 100% needed.

The 12b4 line stage is exactly what I had in mind, I had bought tubes to build one about a year ago and never followed through.

The other one from audioxpress looks like something I need to build for myself!!!

Thanks fellas - Evan
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Old 8th October 2007, 03:07 PM   #5
240z4u is offline 240z4u  United States
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Regarding a transformer..

Most I have been seeing offer 120v out and 6.3v out.

I notice this one you specify has 120/12/12 vac out. I found one at allied but it seems too small.

Any suggestions?

Here is the allied unit

http://www.alliedelec.com/Search/Pro...0973003AE6617F

Evan
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Old 8th October 2007, 05:54 PM   #6
gerryc is offline gerryc  United States
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You can try the Hammond P-T261G6:=: $31 at Antique Electronic Supply www.tubesandmore.com

Secondary: 250 VCT, 130 mA
Filament winding: 6.3 V, 2 A
Mounting centers: 3.56" u-bracket
Weight: 2 lbs.

Should be more than you need. Just use a standard full wave rectifier using the center tap. Same as a bridge on a 125v secondary.
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Old 8th October 2007, 05:57 PM   #7
240z4u is offline 240z4u  United States
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Thanks Gerry.

I actually have a transformer at home I forgot about. I should check the voltages on that.

Evan
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Old 8th October 2007, 06:03 PM   #8
gerryc is offline gerryc  United States
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Ya' know, I forgot you're using 12B4's with a 12 volt filament. My bad. Here's an easy way. Two back to back 12.6 volt transformers, 12.6 volt windings connected together and tap off for the filaments to the 12B4's. 120v in, 120volt out to your bridge rectifier. This is exactly the arrangement used in the AudioXpress article as well. You can get 12.6v 3A transformers from Radio Shack, Catalog #: 273-1511, $10.49 each.
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Old 8th October 2007, 06:23 PM   #9
240z4u is offline 240z4u  United States
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Very interesting way to go about that!

I wonder if Rat Shack has them in stock.


Thanks alot! - Evan
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Old 8th October 2007, 10:13 PM   #10
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Evan,

Stop the head scratching about power trafos. Allied Electronics has exactly what you need.

$11.41 buys stock # 967-9502, which is a Triad N-68X 50 VA isolation trafo. That COMFORTABLY takes care of the B+ requirement.

$8.28 buys stock # 967-2438, which is a Triad VPL24-400 filament trafo with 2X 12 VAC/410 mA. windings. Energizing the 12B4 heaters is WELL taken care of.

BTW, there is GOOD reason to stick with bridge rectified "120" VAC. 600 PIV Schottky diodes are less expensive than the 1200 PIV parts a 120-0-120 FWCT arrangement would use, in order to retain a nice margin of safety.
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