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Alfetta87 20th May 2008 09:56 PM

Need help with a Tube preamp Kit (6Z4, 6N3)
2 Attachment(s)
Hi there,

I need some help, because I bought from eBay a complete cheap tube preamp, using russsian 6N3 tubes and a chinese 6Z4 rectifier tube.
Here is the seller's store, with the kit:

The attached schematic shows to use a 2x230V transformer, with a 0V middle point, to be connected to the ground (not shown on the schematic, but on the PCB).

When I switch it on, the transformer makes some noise, and the resistance RP (1k ohms, 3W) heats up to smoke, and of course I don't have the announced values of voltage.
I wonder what's wrong: the 6Z4 tube says performing up to 2 x 350 volts. See the attached PDF file.

Can somebody help, because the vendor seems to be set on "mute".


Alfetta87 20th May 2008 09:59 PM

Need help with a Tube preamp Kit (6Z4, 6N3)
2 Attachment(s)
The kit says some 5670 tubes could be usec (same pin connection), but actually I installed the 6N3 - anyway, it smokes even without those tubes placed in the socket.

Attached the schematic, if the auction would end.

SY 20th May 2008 11:03 PM

I'd look for a shorted or miswired C7 and/or a shorted or miswired rectifier tube.

d2134 21st May 2008 09:09 PM

C6 is the first capacitor.

SY 21st May 2008 09:48 PM

Yes, C6 then. Thanks.

kevinkr 22nd May 2008 05:04 PM

Hi Alfetta,
Your avatar is the Atomium in Brussels isn't it?

Check for backwards electrolytics at both C6 and C7. (Replace if backwards.)

Look for shorts on the B+ rail - do you have a dvm?

Alfetta87 25th May 2008 08:33 PM

Need help with a Tube preamp Kit (6Z4, 6N3)

Originally posted by kevinkr
Hi Alfetta,
Your avatar is the Atomium in Brussels isn't it?

Yes it is. Because I use to live in Brussels before moving to France. But since I'm not living in or even close to Paris, I'll not put the Eiffel tower... and there is nothing here in Saint Etienne what I could put as a picture!

To all who replied: C6, which has been taken from my box with hundreds of caps, had it's printing almost gone, and I switched plus and minus... shame on me!
Now it's working, at least all the voltages are OK, no smoke and nothing blown away.
This week I'll try the beast on my stereo set...

Thanks for the hints.

- dan

Rewind 21st April 2009 01:59 PM

2 Attachment(s)
I have the "Pre-AMP Amplifier KIT Tube 2*6N3+6Z4 good for DIY"

Have you contacted the seller? He can supply you with a 180V-0-180V transformer. On mine the board says 230V-0-230V but that is way to much. Probably causing the smoking. Be careful. I fried my 700 dollar Lynx L22 soundcard when trying it out, the wrong way, presumably. I still get sound but the soundcard needs 80% on the left volume and 30% on the right channel to give a balanced left/right sound.

I think it happened when figuring out in what order of red white and ground I should have connected the cut XLR cables from the soundcard.

I hooked it up to the soundcard, recording guitar and vocals and I sounded like Elvis. I also used it as a preamp for my monitors. My Alesis M1 Active really came alive. Nick Drake sounds like I am in his bedroom. A proper tube amp can really be a time machine.

Now the only problem is that one of the blue tubes in the middle position is blinking, perhaps indicating something is a bit off.

Study my wiring in my photo and make a guess.

Alfetta87 21st April 2009 02:50 PM

Hi Rewind,

My preamp is complete on one board, rectifier tube, 6N3 tubes and filtacaps everything together.

As said in the thread previously, I made it work correctly, but had a lot of hum problems. I'm using 230-0-230V transformer, and it works fine.

I also had ground problem (causing some hum), now it's on a reasonable level - but still not totally gone.

So my solution is to put the volume pot at the output, then it drives a Luhndal transformer, and output is on XLR. Since the volume pot is always rather turned down, no hum.

Sound is fantastic, the 6N3 is a killer tube. I tried several commercial amps, and none of them could reach the sound quality of this little preamp.

Rewind 22nd April 2009 09:04 AM

Which Pot?
Great that yours is working. Mine is also working, although a little more noise in one channel and of course the stupid blinking of one on the 6N3 driver blue tubes. I can buy another one at this price, anyway.

I found a tube that is american made and is identical to 6N3 except it is supposed to sound much better as a driver tube. They sell these for $7.95 a pair so you can't go wrong. Check Other Items and you will see different kinds. I like the military style tubes. =)

I also want to buy a pot, because a 30W preamp is L O U D. I wonder what kind? I saw a 50W L-pod potentiometer. Is this the right kind? I don't think a 0.02W 100kOhm guitar potentiometer will hold up for the job. Or will it?


To stray off topic, I contacted Lundahl, since I live two hours drive from their HQ *proud* :) , however they said they didn't have my kind of tranformer, sadly. I did however buy their Ribbon Microphone transformer for my DIY ribbon mic with really cool results. I will exchange the guitar magnets for Neodym magnets soon. Money saved! A proper ribbon microphone is expensive!

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