My First Tube Preamp

[Einric]

Hello,

I'm wanting to build a tube preamp to feed my LM4780 Audiosector kits.
I will be paralleling the lm4780s to make mini monoblocks.

I'm needing something with volume control to make sorts of a dual mono mini integrated amplifier.

The project needs to be simple since it is my first tube project.
This leads me to E-Bay of course, I KNOW, I'M SORRY.
Here is what I'm looking at,
Buffer Tube 6N3 HIFI Audio Preamplifier Pre-amp -B3 | eBay
I honestly don't think it will be the best thing since sliced bread but I'm also looking at this like a learning experience.

Any opinions about this item?
Any CONSTRUCTIVE thoughts that might help?

The listing says it requires 150-200VAC, that's no problem but I was wondering what the difference would be with a 200V or a 150V input?

 

[SY]

It's certainly cheap! Without a schematic, it's difficult to say much more. My main worry would be reliability- having a hot rectifier right next to temperature-sensitive electrolytics is not a good idea.

BTW, you're right next to one of my favorite pizza spots ever, over in Belgrade (Damasco's).

 

[jrcanoe]

First let me say I know nothing. I just put togeather built boards and pieces parts in boxes. I am planing to use this preamp with a pair of magic eye tube vu meters because they use the same voltage transformer. I have the two tube version with the solid state rectifier and it does sound good to my 50 year old ears. When I first got it I ran it with two transformers (second one backwards) to give me 140 v ; 10v under spec. Then I got an antek transformer at 160 v ; 10v over spec. I couldn't hear any difference between the two power sources. I use it between my cd or dac and t-amps and I like it cause it seems to smooth the sound and make it not as dry and harsh.

 

[Einric]

SY,

I agree Damasco's is a good slice.

I am looking at taking all of the capacitors and relocating them to the bottom of the board this should help with overheating.
Then I was planning on building a heat shield that is mirror like so I can see the tubes glow from the front of my enclosure.

I'm mainly concerned with voltages and tube types.
Are there any issues with these tubes used?
Are there any drop in replacements that would be better down the road?
Would there be any benefit to using higher voltages instead of lower eg. 200 v.s. 150 or somewhere in between?

Thanks

 

[SY]

Well, the problem is that without a schematic, you can't really say anything about whether the tubes are used appropriately or what the effects of varying B+ are (note that the 6N3 is the rectifier, the ad doesn't say what tubes are used for the audio). You have to trust the designer- and that may or may not be a good idea, there's a lot of really bad Chinese "designs" being sold on EBay for cheap. There may be some good ones, I don't know, but I haven't seen them yet. OTOH, it's $30...

Excellent ideas about thermal management!

 

[Einric]

It looks to me like this kit uses two 6N3's and one 6Z4.

These undoubtedly are Chinese 6N3's is it possible that their naming/numbering scheme is different?

The 6Z4 looks to be the rectifier tube.

 

[dtaylo3]

You may just want to buy the completed amp on their page instead.

It's called the "Finished 12AX7+12AU7 Tube Pre amplifier Preamp"

I would not expect too much, but you never know.

 

[Einric]

I would not expect much either.
BUT.....I'd rather not expect much from $43 and have a learning experience than have a Face->Palm for $200.

 

[SY]

Good attitude.

Is this possibly the same unit?
http://www.diyaudio.com/forums/tubes-valves/123433-need-help-tube-preamp-kit-6z4-6n3.html

 

[Einric]

That setup is very similar but broken into two pieces.
I think I remember reading that post a while back.
If the worst thing I have to worry about on this kit is changing the pot and swapping for 5670's I will be a very happy camper.

 

[Einric]

Now that I think about it more I think I might take this kit and reverse engineer it.
After that I was thinking about rebuilding it P2P.
It appears that this kit is really minimal component count and it shouldn't be too hard to follow the board traces.

 

[tinitus]

Now that I think about it more I think I might take this kit and reverse engineer it.
After that I was thinking about rebuilding it P2P.

then it will be easy to find a better one, with schematic

with a simple, but proven schematic, all you need to study is valve pin orientation, their numbers, and what it means

this way you will also know better how to handle it, in the future

dont be afraid to ask

this thread might be a good start
it is kept very simple
http://www.diyaudio.com/forums/tubes-valves/102352-6v6-line-preamp.html

I was where you are just a year ago, or so
and I remember not understanding one bit about what was what, when looking at a any schematic
but with a bit of help you might learn about the 'symbols' and wires very fast

 

[Einric]

Good Point, any suggestions?

 

[cjkpkg]

Another path that would ABSOLUTELY give you fantastic results is the Aikido. Specifically the Octal or 9-pin All in one kits.

While certainly more expensive that the ebay reference, you will be blown away by the results.

The kits come with very detailed instructions, all the parts to stuff the boards, and support here is widespread - many here have built them and quite a few more than one.

Just add a pot, RCA's, power inlet/fuse, some type of chassis, and a Xformer and you will have a very competent line stage.

AIKIDO

HERE IS MINE

 

[Einric]

This sounds GREAT!!!
I don't mind the extra cost if it is a proven design.
The only choice is what tubes to put in it?
There seem to be many choices and I would be trying to keep my total budget under $300 including X-Former, RCA's, selector switch & chassis (wood).
Any opinions on tube selection & other options?

 

[cjkpkg]

I would say the 9 pin allows for more tube rolling...things like the 6CG7, 6DJ8, 6N1P, 6922 and so on allow for a lot of tweaking and experimenting. The only thing you need to really keep an eye on is the cathode resistor to keep the current right for each tube type.

All these tubes are readily available. A Hammond or Allied Electronics Xformer should be readily available. Edcor is what I typically use but lead times are long ~ 6 weeks at best.

Last is the chassis...you can use a Hammond aluminum from Mouser or Digikey or go whole hog with a HiFi2000 (modushop.biz) type.

Aikido All In One Manual

Check out the manual...it gives lots of info on different tube types and operating points...

 

[Einric]

I will read through the manual and see what it looks like.
I'm looking at this transformer, it is a 230x2 6.3x2. I was figuring on running the 6.3 secondaries in series to get the 12v the design needs.
I am a pretty good woodworker and I have an aluminum supply place nearby so I will be building a very nice chassis myself.

 

[tinitus]

decide on a design before choosing transformer

I would suggest your 'keyword' should be cathode follower

if you look at a schematic, its the ones where output is taken from 'bottom' of the tube(cathode)

 

[Einric]

BTW what is the difference between the 12AX7 & the 12AX7-T?
I'm trying to come up with a build that will have the availability to use high grade tubes that seem to be harder to come by (PSVANE & Treasure Tubes), they might be nice down the road when I get more comfortable with tweaking the preamp.

I did notice the voltage B+ suggestions based on tube type.
This led me to think of a 200v transformer for a 280v B+ to run a 12AX7 and quite a few others.

 

[evanc]

The akido kit is a great choice. It leaves enough to the builder to make it a fun build without leaving too much to chance.

I like mine with 8cg7 tubes real cheap and the right amount of gain for me.