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    Building, troubleshooting and testing of these amplifiers should only be
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    the safety precautions around high voltages.

6V6 line preamp

Hi Salas,

I did trip over your other thread regarding this one and inmediatly called my attention since I started to gear up for yet another tube preamp to replace the 417A that I now use which has two shot fallings, namely hi gain and hi zout used as a CC - plate follower.

Being an addict for simple topologies I had decided to build the 12b4 that has a good reputation from this thread http://www.diyaudio.com/forums/showthread.php?s=&threadid=68267&perpage=25&highlight=&pagenumber=1 but also was thinking on Geek’s permutation which shows an even lower zout, not that I need that since I’m driving a low capacitance 47k amp input.

I’m at the very beginning of this project, have ordered only the tubes so nothing stops me from changing to this very interesting line stage you have concocted.

Had you have a chance to compare the 12B4 to your 6V6? Would love to here some comments to this regard.
 
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Yes, I have tried 12B4 in simple triode once. But I did not choose it as a keeper. Its precise, has impact, but its cold in my opinion. Reminds me of solid state sound somehow. This 6V6 circuit has flow and musicality, along being really big and powerful. If you can accept that it is more easy to pick up vibrations, or that some brands do expand their metals during warm up and you can hear some ding dong sounds for 2-3 minutes, I must say this is the best valve preamp I ever had by a very good margin. If you start from scratch, I highly advise you to build a sub chassis for the 2 6V6s that may be elastically decoupled from the rest of the chassis. This will save 80% of the microphonics. Another good thing with my 6V6 circuit is that it displays very practical gain for modern sources. You get a good rotation range on your volume pot. Driving tube amps or 50k input SS amps is no problem. I have tried many configurations with noval follower output stages that have better Z out on paper, but the 6V6 with 20mA driving an amp, puts you in another league.
 
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If you triode connect an EL84 its rp drops to around 2k. If you wire it as a cathode follower, I would expect it to fall 10 times lower circa 200 Ohm. But why adding a second stage with 100% local feedback? It will steal the natural tone and the immediacy. The 6V6 here has 1.5k Zout and it drives much a bigger sound in the power amps than CF stages I had used before with 3 to 5 times lower Zout.
 
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mod_evil said:
Its a good idea use local feedback.

I'm thinking to use ECL82 with the Pentode conected in TRIODE MODE, to drive a SENNHEISER HD580.

Best Regards,
Felipe Navarro.


In such a case I would try it. If you go down to 100-200 Ohm with such a cathode follower you may just lose 25-35% of your headphone amp gain when driving the HD 580s. Not too bad when gain is excessive anyway if there is a triode stage before the CF driven by a CD player.
 
salas said:
I have tried many configurations with noval follower output stages that have better Z out on paper, but the 6V6 with 20mA driving an amp, puts you in another league.


Nice to hear your comments on this one, thanks. Ok, so 1.5 K zout, good to drive a 20K load? Or this would be asking too much like on my 417A? - anyway I think the 417 will be similar to the 12B4 sound wise even though it has a somewhat lower Zout as a CC which is not what I’m looking for, I would prefer something like the 6V6 character you described.

Wonder also, if bass gets too rounded, meaning if attack gets somewhat sluggish?
Did you use AC for the heater?

Sorry, to many questions but I has looking to the newer issues from Sovtek’s or JJ 6V6 seems to have ribbed plates, do you have any experience with those?
 
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apassgear said:



Nice to hear your comments on this one, thanks. Ok, so 1.5 K zout, good to drive a 20K load? Or this would be asking too much like on my 417A? - anyway I think the 417 will be similar to the 12B4 sound wise even though it has a somewhat lower Zout as a CC which is not what I’m looking for, I would prefer something like the 6V6 character you described.

Wonder also, if bass gets too rounded, meaning if attack gets somewhat sluggish?
Did you use AC for the heater?

Sorry, to many questions but I has looking to the newer issues from Sovtek’s or JJ 6V6 seems to have ribbed plates, do you have any experience with those?

- 1.5kX10=15k < 20k = OK (Better than ten fold). 22ma of cathode current counts much more in my experience vs lower Zout from a CF working with 5-10mA.

- I was listening to Pictures At An Exhibition (Wiener Phil. / Previn) just before. The double basses and tympani in The Great Gate Of Kiev left me speechless. The attack and body was elastic, majestic and instantaneous.

- LM317T current sourced heaters. One per 6V6, dedicated secondaries, bridges and caps.

- JJ's I had and eye for them, seem rugged. I did not get on a couple yet. Its I am spoiled with excellent NOS I have. I still wanna see how it does for microphony.
 
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salas said:


<snip>

- I was listening to Pictures At An Exhibition (Wiener Phil. / Previn) just before. The double basses and tympani in The Great Gate Of Kiev left me speechless. The attack and body was elastic, majestic and instantaneous.

<snip>


One of my favorite works, I've got at last count at least 8 different recordings of it on vinyl and cd..


:D
 
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Rafael Frühbeck de Burgos, 1965 Abbey Road EMI. Lucia Popp, Gerhard Unger, Raymond Wolansky, John Noble. I got it on LP when I was 12. I still have it. Best Carmina I ever got.
 

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Well… I’m committed, just ordered from The Tube Store a matched pair of JJ 6V6S and a JJ GZ34S/5AR4 companion tube rectifier.

This will be my first project with a tube rectifier, PSU will be similar to Salas posted schemo, iron will be made by yours truly and I have sufficient caps for the project except the cathode by pass for which plan to order some Nichicon Muse GZ and Kiwame resistors to cover those critical places.
:idea: :yes:
 
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You will lose 20-30V against the bigger types. But if you make the chokes bigger, like 25H, and lighten the caps to say 100uF, your voltage will be ok, and you wont be pounding the 5Y3 with big reverse current. Anyway, you are bound to roll rectifiers for other reasons... Namely tone preference, and system tone matching. The rectifiers are very much audible, especially in this preamp. Also use a 15W bleeder to ground after the first choke. Roughly as many H, so many kOhm. That will regulate the choke. Sounds quieter.

P.S. Since you are making the iron yourself, why not upgrade the trafo to say, 2X320VAC, so you can have spare voltage for 25H-30H chokes and different rectifiers? You can always trim the voltage with a series resistor right after the rectifier. That way, hum will stay low always.
 
I was drooling over the Tubes Gallery thread an run over some very nice gear posted by Teablue (from Saigon) which has been the only contribution to DIYaudio

http://www.diyaudio.com/forums/showthread.php?postid=1147029#post1147029

within about 6 projects he shows a 6V6 preamp with 5Y3 but no other information regarding operating point or B+ nor other PSU details. I PM him to see if he could share the schematic with us.

Thanks Salas for the previous post with lots of useful information, BTW it was not very clear in your schematic the amount of capacitance used at the input of the PSU filter was that 2X 2.2uf after the rectifier? I know one can play with it to adjust the voltage but was curious enough to ask.
:cool:
 
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Correct. 2,2uF//2,2uF PP 630V.

Follow the basic circuit, and do not worry much about the exact supply values. The main thing is to keep it valve rectified, with at least one central and two CLC branches. The more you can increase L and moderate C without getting hum, the best the sound. It depends on your speakers sensitivity as well. The more sensitive they are (revealing noise more), the more heavy handed you go on the supply hum rejection values.