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My Mini 12AU7 Tube Preamp
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Author:  CrazzyAbtTubes [ 30 Jul 2009, 14:48 ]
Post subject:  Re: My Mini 12AU7 Tube Preamp

The RCA circuit is actually quite different in terms of the values used, it runs off a 250V supply the GE is only like 150, which is the same voltage as the Mini Preamp, but I was going to upgrade the power supply anyways even though this is supposed to be a simple budget project. ;)

I will have to finish the GE schematic and see how it compairs with the RCA one. :)

Author:  keaster [ 06 Aug 2009, 21:50 ]
Post subject:  Re: My Mini 12AU7 Tube Preamp

Quick question, I just finished the power supply for this bad boy and when I test the high voltage I get about 180vdc, is this right? or am I going to blow something up?

Author:  Gio [ 07 Aug 2009, 12:11 ]
Post subject:  Re: My Mini 12AU7 Tube Preamp

Did you measure the power supply while it is under load? If not, if will likely be lower when loaded. The tubes will not have any problems with that voltage. How much gain did you need from the preamp?

Author:  keaster [ 07 Aug 2009, 16:12 ]
Post subject:  Re: My Mini 12AU7 Tube Preamp

no I have not checked it underload because I wanted to make sure the PS was ok before connecting, as for gain i'll be using it for an ipod

Author:  CrazzyAbtTubes [ 07 Aug 2009, 19:58 ]
Post subject:  Re: My Mini 12AU7 Tube Preamp

Well there keaster, If you got the recommended transformer size (120V at 25-45MA) than you are all set to go, at 180 volts you must have a good power supply. :thumbsup:

I think you may be in for a suprise when you get your preamp working, it sounds great! with the ipod. :headphones:

I will be making an updated phono stage to go along with this preamp later after I finnish rebuilding the C8 preamp, and that Light speed Attenuator would go great with this preamp if you decide not to use a volume control like the way mine is.

Author:  keaster [ 10 Aug 2009, 14:40 ]
Post subject:  Re: My Mini 12AU7 Tube Preamp

Ok so I have finished the pre amp and hooked it all up. You were right this little guy has alot of gain Im using the 100k pot for volume and have it hooked through a ipod dock. So the problem im having is a very noticeable hum or sort of static. Ive gone over things looking for errors and I cant find any. Is there any things to look at first or things to check. I didnt use point to point I used a prototyping board and ran short wires from the board to the tube sockets. Also the power supply is in the same box, I used a bit bigger box then yours. If you can help any that would be greatly appreciated.

Author:  CrazzyAbtTubes [ 10 Aug 2009, 21:46 ]
Post subject:  Re: My Mini 12AU7 Tube Preamp

If it's like a 60hz type oscilation noise... remove the volume control pot and try the ipod strait in to the preamp, I have had many issues with volume controls lataly,not sure why I just get allot of noise, which is the main reason why there is no volume control on mine, but also the ipod all ready has one. :)

Well now that you have your preamp constructed and working I would really like to see it, ither by posting some pics here or, if that is too much time you could always send them to me and I could scale them down andpost them for you, it would cirtainly give me a reference to any other problems that could occur. ;)

Also be aware that I will be making an update in some time to come, all new power supply and a "MINI"phono stage that has been much requested.

Author:  Meltz014 [ 19 Aug 2009, 21:16 ]
Post subject:  Re: My Mini 12AU7 Tube Preamp

Hey i have another question.

In the schematic for the amp part, on the smaller circuit to the right, those nodes labeled 4 and 5, do they just go to pins 4 and 5 of the tubes? Thanks

Author:  Gio [ 20 Aug 2009, 20:58 ]
Post subject:  Re: My Mini 12AU7 Tube Preamp

Yes, pins 4 and 5 are the heaters pins for the tube.

Author:  ronp [ 22 Sep 2009, 18:13 ]
Post subject:  Re: My Mini 12AU7 Tube Preamp

Just starting on the "mini pre amp" and I have two questions. YEARS ago I used to work on tube equipment and have forgotten just about EVERYTHING, question one; since this a modification of Tim Williams amp I'm using his label; what does C2 do in the circuit and second, how did you come up with the capacitor values which are so different than his? Thanks very much Ron Parola

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