Vademeccum pre.

[ruffrecords]

[quote author="Kit"]www.munchenmotor.se/mickesbilder/ef86pre.jpg

Comments and critisism are most welcome.[/quote]
I too would be inclined to use the EF86 as a pentode for the extra open loop gain. In a preamp, the signal level at the EF86 anode will be pretty low so distortion is not going to be such a big issue. Wiring it as a triode reduces open loop gain which increases distortion unless you particularly prefer the triode sound.

I am intrigued by the many various ways of achieving a balanced output and it occurs to me in many situations it is not really necessary. I am working on a very similar design with one half of the ecc82 as a triode followed by the other half as a cathode follower for a low impedance but unbalanced output. It also provides a nice low impedance source for the feedback network.

Ian

 

[Guest]

[quote author="Kit"]BTW, do any of you guys know of a source for cheap power transformers for tubes?

I need a secondary winding around 280-300V with a 50V tap + a 6,3V heaterwinding.

Thanks.[/quote]

I have some,

dual 120V primary with secondaries of 270-0-270 500mA, 12.6v CT 2.5A, 12.6v 2.5A, 60V 50mA

 

[Kit]

In a preamp, the signal level at the EF86 anode will be pretty low so distortion is not going to be such a big issue.

I agree, but distortion is not what im afraid of here. If that was the case i wouldnt have buildt a tube pre to start with. :green:

Partition noise is what im worrying about, but maybe it wont be that bad. I havent crunched any numbers yet.

About the power iron, gentlemen im very gratefull for your suggestions but i have decided to scrap the idea i had about the 50V tap.

I do have a cheap source for "regular" tube power iron without the tap so im just going to order those.

But thank you. :thumb:

 

[ruffrecords]

[quote author="Kit"][Partition noise is what im worrying about, but maybe it wont be that bad. I havent crunched any numbers yet.:[/quote]
As a rule, with a transformer input, it is only necessary to design the input circuit so that the source impedance reflected into the grid is preferably four times the tube noise resistance. Tube noise will then have a negligible effect on total noise.

Ian

 

[ioaudio]

prior to building i was reading about partition noise and that was the main reason why i added the triode/pentode switch on the pre#7. i just left it in, and very often prefer high gain pentode mode, while turning down the output pot.
-max

 

[Kit]

This past week i took the time to wire the EF86 as a pentode with a gain of approx. 100.

Noise to my ears wasnt to bad.

I also scrapped the idea about the gain/nfb switch. A log pot will do fine as long as youre not tracking stereo.

I will do an update on this with schematics, pictures and hopefully some specs (thd and s/n ratio etc.) when all 4 channels are built.

Take care.

/Mikael.

 

[Kit]

2 channels done:

www.munchenmotor.se/mickesbilder/front.JPG

www.munchenmotor.se/mickesbilder/insida.JPG


2 to go.

I´ve done some changes to the schematics I posted earlier, I will try to post the new schem. as soon as possible.


And finally, remember to brush your teeth kids.
Dental hygien is important.
Dont let the English tell you any different.:razz:

:shock: :green:

 

[rotation]

Did you do any recordings with your new Vademecuum pre? :grin:
How does it sound? Can you hear the difference between penthode and triode mode?
You didn't tell which transformers you are using..

 

[Kit]

Im not really into describing sonics, since it is after all a very subjective matter.

But if I must say something, it would maybe be "smooth".

The trafos are OEP (input) and Edcor (output).

Vademecuum pre

Hey, I kinda like that name. :thumb:
Very clever.

 

[rotation]

[quote author="Kit"]

And finally, remember to brush your teeth kids.
Dental hygien is important.
Dont let the English tell you any different.:razz:

:shock: :green: [/quote]

Well, you gave me suggestion for the name. Vademecum is actually a brand of tooth paste. So if you call your preamp Vaccum i call it Vademecuum :green:

Did you noticed any difference in sound between pentode and triode mode? Does it make any difference in bass, mids or highs? Which mode gives more of that tube sound? Sorry for this difficult questions, but i'm very interested in knowing what the differences are.
I actually built ioaudio's #7 preamp, but it ended as a tube DI and i never had a chance to do detailed tests of mic input.

 

[Kit]

Did you noticed any difference in sound between pentode and triode mode?

Not much really, but I havent compared them side by side as a true A/B test.
I dont know what your criteria for a "tubey" sound is (theres that subjective thing again) but maybe the triode mode gives an even softer sound. But its really hard to tell.

Sorry for this difficult questions, but i'm very interested in knowing what the differences are.

Nuemas problemas.

Why dont you build it so we/I can get an second opinion.

Its starting to feel like I am talking to myself.

Note to self:
Dont insult a whole nation of people on public forums.

 

[margot]

Man that pre looks cool. I want to start building stuff for my studio. Was this a pretty straightforward build as far as they go? Not a lot of experience with this stuff but I want to build a decent front end before hitting pro-tools. Right now I'm using Grace pre's and they are very "clean" "neutral" "sterile". Jeez- talk about subjective terms. Anyway not sure if this will get me the warmth I'm looking for but I thought I would ask.

Thanks,

Margot

 

[Kit]

Hi margot.

Man that pre looks cool.

Thank you. :thumb:

Note that in the recent pics the input XLR´s is wired wrong.................
Too much soldering and too little sleep, I guess.

About the sound..............I dont know how to describe it. Its nice.

Im currently waiting on a RME fireface soundcard that i ordered some time ago. When I get that linked up properly to my laptop I should be able to make some recordings. Also the schematics are slightly revised from the original, I should upload that to.

Damnit, so little time. :evil:

Was this a pretty straightforward build as far as they go

I would say so. Its an easy circuit. Not much to screw up. With a proper layout you shouldnt experience any problems with oscillation and such.

 

[margot]

Very cool, thanks indeed!

Margot

 

[SonsOfThunder]

Just curious,

Which input trafo model is that?

What brand of EF86 are you using and did you have to sort for noise? There was a big problem with that tube for a while from some manuf as I have heard.

Nice build! Makes me want to finish my stinkin' unfinished pre!

Peace!

 

[Kit]

Thanks.

The input is a OEP X187B. Fairly cheap centertapped primary 600 : 50k thing.

The EF86 is TAD branded. Dont know which factory it comes from. It is supposed to be selected for low noise. But who knows, really.

I dont experience them to be noisey, though.

 

[CJ]

EF86 noise half life is about six months.
If you make it that far, your set for a while.

You can prolong life by dropping screen volts down to Sevetlana EF86 specs.

 

[SonsOfThunder]

Thanks for the tips, Ceej!

 

[Kit]

Behold, the mighty "Vademeccum" pre:

Comments and critisism will be ignored. :razz:

 

[SonsOfThunder]

I like it. Why don't you change the thread title to the name of your pre?

Thanks for sharing! I'm still trying to figure out how to get those input transformers here in the states. They don't show up on the InOne network like the rest of the OEP stuff.