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PostPosted: 02 Sep 2008, 23:26 
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Has anyone tried either of these preamps ? the 6SN7 design looks nice and simple...
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-Dan


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PostPosted: 02 Sep 2008, 23:56 
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Well, I have not tried those exact values, but those are both SRPP circuits which have a decent following.

While I have not done the math I can tell you that amount output capacitors on Pat's version are absolutely ridiculous. 0.47uF is overkill, not to mention 50uF!


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PostPosted: 03 Sep 2008, 07:27 
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Hi Dan, welcome to the forum!

As noted by Bill, those are SRPP tube preamp circuits. Here are some notes for a 6DJ8 / ECC88 SRPP Tube Preamplifier which I checked on TubeCAD, but I don't recall distortion values. I agree, you don't need 50uF for the output.

Also, Bruce's OddWatt projects use SRPP driver stages. Take a look at the 12SL7 SRPP / KT88 Push-Pull DIY Tube Amp for more ideas

Hope this helps,
Cheers.

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PostPosted: 03 Sep 2008, 19:09 
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thanks for the info guys,

ps gio the circuit you posted looks even simpler might be a goer when i get round to finally making a tube pre.

-Dan

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PostPosted: 03 Sep 2008, 22:03 
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Yeah, it is simple and supposed to be pretty good. I did that up in TubeCAD, so I can recalculate a different gain if you so desire. What I really want to try with it is an LED on the cathode instead of a capacitor and a resistor.

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PostPosted: 04 Sep 2008, 21:24 
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I build the second circuit using 7n7 (loctal socket version of the 6sn7).

It is one of my favorite preamp now, highs are dynamic, vocals are great, bass is very tight. I think this circuit is quite neutral, but not as neutral as my 12b4 preamp.


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PostPosted: 04 Sep 2008, 22:31 
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That looks easy to make and I have a few of those tubes here and probly build a nice power supply out of stuff I have here. Just need to get some caps, got everything else.


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PostPosted: 04 Sep 2008, 23:03 
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Hi Endspec, current draw will be small so you don't a large transformer or all that much capacitance.

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PostPosted: 06 Sep 2008, 10:06 
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gmilitano wrote:
Hi Endspec, current draw will be small so you don't a large transformer or all that much capacitance.

I am using a 76mm cored transformer because I have it on hand. But a 230-0-230 secondary with around 40ma should be enough for the second circuit. I used a 10H 80ma choke on the PSU, and did a CLCRCRC filtering which reduced the ripple to less than 100uV if I remember right. I used 6X5 rectifier. The last capacitor on the PSU is a 10uf polypropylene, and I did a "pseudo-mono" on the PSU, after the choke on the PSU and the capacitor, I use two resistors to branch the PSU. I don't have the preamp with me at the moment, but after the 6x5 rectifier, the first cap is a 4.7uf MKP -> 10H 80ma choke -> 100uf rifa electrolytic -> branch into two rails using 100R resistors into 100uf rifa -> 100R -> 10uf MKP.

A friend is using a regulated 250V supply and it sounded a bit better than my unregulated supply. My friend is using SS regulation on his PSU.


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PostPosted: 07 Sep 2008, 00:42 
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Well I don't need to build a power supply, my rca sp-20 has a pre-amp power plug built in with filiment and b+ and b- allready. but I was looking at the caps in the 6sn7 pre and do they need to spec so high on voltage, the 1 uf @ 630 volt and the 470 uf at 630 volt could you lower the voltage obn those to like 400?


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