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PostPosted: 17 May 2009, 00:16 
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Joined: 15 May 2009, 12:21
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Hi, Sorry to come across as impatient, I'm not. I'm grateful you help me at all.

Do you know if 120-0-120 deal is enough to crank up a 6x4? I've get some of these that are good, Mullards I think. I got them for my HF-81. I can't remember what the operating points are for them.

Thanks again for taking your time.


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PostPosted: 17 May 2009, 11:10 
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Joined: 28 May 2008, 21:53
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Location: Winnipeg, CANADA
Yes, the 6X4 will work, but you would get more voltage using solid state rectifiers.

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PostPosted: 20 May 2009, 16:02 
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Location: Littleton, Colorado
What's the gain on this little guy? Would it be suitable for a record player/turntable?


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PostPosted: 21 May 2009, 14:52 
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Joined: 14 Oct 2008, 17:35
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Location: Dartmouth, Nova Scotia
tmoble, that transformer would work perfectly using a 6X4, and you can use the traditional half/full wave bridge setup, but if it came out the HF-81 I would suspect that it would be of higher voltage for that amplifier, current output would be over kill for this project... :hot:

As for the gain of the preamp, it might be enough for phono but does not have the RIAA type equalizer in it needed to get good low frequencies of an LP, and I didn't design this circuit I just modified it to suit the output of my ipod.

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PostPosted: 14 Jun 2009, 18:43 
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CrazzyAbtTubes,
or/also is it Alan? well first off thanks for this project document! The question I would like to ask you at this point, is what the practical differences are between this, the "DIY 12AU7 Tube Preamplifier Project" and the lesser gain-ing "12AU7 / ECC82 Cathode Follower Tube Preamp Schematic". I understand the concept of more gain/ less gain between the models, however, do either models have a variable gain pot? / could have one? I was thinking that if the "only" difference between these pre's were the respective gain levels, that then, the high-gain model would be a better choice, i.e. useful for more applications, given that the gain could be adjusted.

Maybe you can bring light to any features/implications that I have overlooked? I am still not wholly framiliar with the subtlties of these units... however; the main purpose I am considering one is to boost the signal coming from the Tape Out(s) in cassette multitracker, which would then be sent to a mixer (at line-level?) and then AD. If it helps: the output load impedence of the Tape Out is 10k ohms +, and the normal output level is -10 dBV.

I appreciate you help!


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PostPosted: 15 Jun 2009, 15:55 
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The Mini 12AU7 Tube Preamp has about triple the gain, about 25 versus the 12AU7 / ECC82 Cathode Follower Tube Preamp Schematic which is about 8.
Cheers

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PostPosted: 16 Jun 2009, 20:42 
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I realize I was a bit slow with answering your questions but anyways, in the original schematic ther is a 100K pot at the input for gain control, as for the gain Gio being on the ball here has givin the over all gain of the unit, which I cannot recall what it really was, I just knew it was allot, and even more with the voltage I used.

It is not a bad idea to do what I did and bread board the preamp befor actually building it, I originally had a volume control and had no problem using a cd player, I removed it because the ipod all ready has one, and it being very small I called it the Mini Tube Preamp, and one of these days I may try a 6CG7 in place of the 12AU7 and see if there is any improvement in sound.

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PostPosted: 16 Jun 2009, 22:40 
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Hi Alan,
To avoid confusion, the gain of the preamp remains fixed. What the potentiometer (volume control) does is attenuate the signal.
Cheers

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PostPosted: 30 Jul 2009, 10:22 
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I have received a few questions about using this preamp as a phono stage, but I am afraid the answer is NO! I am in the process now of creating a Mini Tube Phono Preamp right now using the schematic from an old GE tube manual. :idea:

The circuit is of the same simplicity and power requirements just with an RIAA equalizer, so STAY TUNED! ;)

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PostPosted: 30 Jul 2009, 12:17 
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Hi Alan, here is a real simple RIAA circuit from the RCA manual with one tube per channel: 7025 / ECC803S / 12AX7A Tube Phono Preamp Schematic.
Cheers

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